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it may be worth recalling some of the main properties of the milky way open cluster system . the census of open clusters is still highly incomplete beyond distances of a few kpc from the sun , although the situation is improving with new surveys such as 2mass ( see e.g.  the contributions by carpenter and hanson in this volume ) . the luminosity function of milky way open clusters was analysed by @xcite , who found it to be well modelled by a power - law @xmath1 over the range @xmath2 . however , they also noted that extrapolation of this luminosity function would predict about 100 clusters as bright as @xmath3 in the galaxy , clearly at odds with observations , and thus suggested some flattening of the lf slope at higher luminosities . the brightest known young clusters ( e.g.  ngc  3603 , @xmath4 and @xmath5 per ) have absolute @xmath6 magnitudes of @xmath7 , corresponding to total masses of several thousand . recently , there have been claims that the cyg ob2 association might be an even more massive cluster @xcite , but this object is probably too diffuse to be a bound star cluster ( though it does have a compact core ) . there are , however , a number of old ( @xmath8 gyr ) open clusters in the milky way with masses of @xmath9 @xcite . these objects are likely to have lost a significant fraction of their total mass over their lifetimes , and may thus originally have been even more massive . they serve to illustrate that , even in the milky way , the distinction between globular and open clusters is not always clear - cut . it has been recognized for about a century that the magellanic clouds , and the lmc in particular , host a number of `` blue globular clusters '' @xcite . among the most massive of these is ngc  1866 , with a mass of around @xmath10 and an age of @xmath11 myr @xcite . an older example is ngc  1978 with similar mass but an age of 23 gyr , clearly demonstrating that at least some such clusters can survive for several gyrs . the interaction between the lmc and the milky way has probably affected the star formation history of the lmc , which is known to be bursty with major peaks in the star formation rate correlating with perigalactic passages @xcite . one might argue , then , that the formation of ymcs in the lmc could be induced by interaction with the milky way . however , the lmc is not the only example even in the local group of a galaxy that hosts ymcs . another well - known example is m33 , which does not display evidence for a bursty cluster ( and , presumably star- ) formation history @xcite . @xcite have identified many more star clusters in this galaxy , though not all are particularly massive . with the launch of hst it became possible to investigate more crowded and/or distant systems in detail and attention started to shift towards more extreme starbursts , including a large number of merger galaxies ( e.g.   whitmore , this volume ) . it is now clear that luminous , young star clusters often form in very large numbers in such galaxies , and this has led to suggestions that formation of `` massive '' star clusters might require special conditions such as large - scale cloud - cloud collisitions @xcite . however , the question remains to be answered why some non - interacting galaxies also contain ymcs , whereas apparently the milky way does not . ymcs are now being found in an increasing number of non - interacting galaxies , posing a severe challenge for formation scenarios which require special conditions . during the 1980s , some studies had already identified ymcs in a few galaxies beyond the local group @xcite . we undertook a systematic , ground - based study of 21 nearby spirals , aiming at identifying cluster systems and further investigating which factors might lead to the formation of ymcs @xcite . generally lacking sufficient resolution to identify clusters as spatially resolved objects , our candidate lists were compiled based on @xmath12 photometry , selecting compact objects with @xmath13 and @xmath14 brighter than @xmath15 ( for @xmath16 ) or @xmath17 ( for @xmath18 ) . we also required that the objects had no h@xmath19 emission . the @xmath20 limit excluded most foreground stars , while the @xmath14 limit was designed to minimise the risk that individual , luminous stars in the galaxies would contaminate the sample . as the mass - to - light ratios of star clusters are highly age dependent , the magnitude cut does not translate to a well - defined mass limit , but most clusters selected in this way have masses @xmath21 . our survey would probably pick up a few clusters in the milky way . in the lmc , 8 clusters in the @xcite catalogue pass our selection criteria . we found a surprising variety in the numbers of ymcs in the galaxies . some galaxies , such as ngc  45 , ngc  300 and ngc  3184 contained hardly any clusters passing our selection criteria , but in others we found more than a hundred . the two most cluster - rich galaxies were ngc  5236 ( m83 ) and ngc  6946 , both of which are also known for their very high supernova rates and surface brightnesses , indicative of very active star formation . following @xcite , we defined the _ specific luminosity _ of young star clusters as @xmath22 where @xmath23 and @xmath24 are the total @xmath25-band luminosities of clusters and their host galaxy . the @xmath26 turned out to correlate strongly with the host galaxy area - normalised star formation rate @xcite , as if bound star clusters form more efficiently in higher - sfr environments . here , it is important to note that our sample excludes the very youngest clusters , which are often located in crowded regions in spiral arms where they are difficult to identify with ground - based imaging . therefore , it is probably better to think of @xmath26 as a _ survival- _ rather than a formation efficiency . in fact , most stars probably form in clusters , both in normal galaxies such as the milky way ( * ? ? ? * carpenter this volume ) and in the mergers like the antennae ( fall , this volume ) . the fraction of those clusters which remain bound may vary , however .    while @xmath26 may be a useful measure of the overall richness of a cluster system , it does not provide any information about possible variations in the cluster mass distributions , and in particular , whether some galaxies form a higher proportion of _ massive _ clusters than others . this question still remains largely unanswered , because mass distributions are difficult to derive observationally . in order to convert observed cluster luminosities to masses , the m / l ratios need to be known . these , in turn , depend strongly on the cluster ages , which can not be reliably estimated without photometry in ultraviolet passbands . this is mostly a consequence of the fact that the colours of young clusters are dominated by hot stars , where the rayleigh - jeans approximation applies for passbands centered at optical wavelengths . a `` poor man s '' solution is to look at luminosity- rather than mass functions , bearing in mind the first are not necessarily identical to the latter .    in @xcite , archive hst data were used to analyse the luminosity functions ( lfs ) of star clusters in 6 nearby spirals which had previously been studied from the ground . the lfs were generally found to be consistent with power - laws with slopes between @xmath27 and @xmath28 , somewhat steeper than the value found for milky way open clusters by @xcite , though not extending nearly as deep . for young clusters in m66 , @xcite found a lf slope of @xmath29 . if the luminosity function has a universal power - law form which is populated at random , one would predict a strong correlation between the total number of clusters in a galaxy and the luminosity of the brightest cluster . such a relation is indeed observed and has a slope , normalisation and scatter similar to those expected from sampling statistics arguments @xcite . however , it should be noted that a few galaxies do stand out , having clusters that are much too bright for the total number of clusters in those galaxies . notable examples are ngc  1569 and ngc  1705 , both of which are dominated by 12 highly luminous clusters . the possibility remains open that these clusters formed by a special mechanism , but the issue needs to be investigated in more detail . hst archive data for a larger sample ( 17 ) of nearby spirals were analysed in @xcite . the main aim here was to study the structural parameters and investigate possible correlations with age , mass or other cluster properties . the clusters were modelled using ` eff ' profiles of the type shown by @xcite to fit lmc clusters : @xmath30^{-\gamma/2 }    \label{eq : eff}\ ] ] structural parameters were obtained from fits to wfpc2 images and combined with @xmath31 photometry from ground - based imaging . [ fig : reff_mass ] shows the cluster half - light radii ( ) versus masses estimated from bruzual & charlot ssp models . only clusters with @xmath32 are included in this plot , as  is undefined for @xmath33 . the majority of clusters have half - light radii of 34 pc albeit with a fairly large scatter . interestingly , this is similar to the effective radii of galactic and extragalactic old globular clusters . the dashed line shows the relation corresponding to a constant cluster density ( @xmath34 ) , while the solid line is a least - squares fit to the data . no strong correlation between  and mass is observed . a formal fit to the data yields @xmath35 , similar to the @xmath36 relation found for young clusters in the merger remnant ngc  3256 by @xcite . the observation that no strong correlation exists between cluster size and mass implies that high - mass clusters generally have much higher stellar _ densities _ than low - mass clusters , a fact that may have important implications for theories for cluster formation . if the cluster ages are known , luminosities can be converted to masses using simple stellar population models and assuming a stellar initial mass function ( imf ) . an alternative approach is to obtain dynamical mass estimates by measuring the internal velocity dispersions and cluster sizes and applying the virial theorem . the dynamically derived m / l ratios can then be compared with ssp models for different imfs , providing a potentially useful method to constrain the imf . the line - of - sight velocity dispersion @xmath37 , mass @xmath38 and projected half - light radius  are related as @xmath39 where @xmath40 . in practice , however , there are many caveats to this method , both theoretical ones ( assumption of velocity isotropy , virial equilibrium , effects of mass segregation and binaries ) , and practical ones : for a mass of @xmath10 and @xmath41 pc , the line - of - sight velocity dispersion is less than 4 km / s . the red supergiants which provide most of the lines useful for velocity dispersion measurements have macroturbulent velocities on the order of 10 km / s , with a scatter of perhaps 12 km / s @xcite . since the velocity dispersions usually have to be derived from integrated light , it is clear that this method is limited to relatively massive objects . even if spectra of sufficient resolution to resolve the line broadening ( @xmath42 ) and s / n could be obtained , an exact match between the template stars used to derive the velocity dispersions and those present in the cluster becomes increasingly critical as the cluster mass decreases . additionally , the clusters have to be close enough that reasonably reliable size estimates can be obtained , although these are less critical since @xmath38 scales only linearly with . several groups have obtained dynamical mass estimates for extragalactic young star clusters , sometimes with hints of non - standard imfs ( e.g.  @xcite ; see also mengel , this volume ) . in our sample of spiral galaxies , we found a few clusters for which dynamical mass estimates appeared feasible . critical selection criteria were that the clusters be reasonably well isolated , so that the spectroscopic observations would not be contaminated by neighbouring objects , and that they have hst imaging for reliable size measurements . objects which satisfy these criteria include one cluster in ngc  6946 which was observed with the hires spectrograph on the keck i telescope @xcite , and two clusters in ngc  5236 ( fig . [ fig : cl5236 ] ) , observed with uves on the eso vlt @xcite . structural parameters are available for all clusters from hst imaging , and ages and reddenings were estimated by comparing ground - based @xmath31 colours with bruzual & charlot ssp models . basic properties for the three clusters are summarised in table  [ tab : cprop ] . they all have masses greater than @xmath43 , well in excess of those of the most massive young lmc clusters . even if our reference frame had been the lmc rather than the milky way , we would still have characterised these clusters as `` massive '' . in fig . [ fig : pmtol ] , the observed @xmath6-band m / l ratios are compared with ssp model predictions for various imfs . these ssps were computed by populating stellar isochrones from the padua group @xcite according to the imfs indicated in the figure legend , i.e.  a @xcite imf and @xcite - type imfs with lower mass cut - offs at 0.01 , 0.1  and 1.0 . the kroupa imf nominally extends down to 0.01 , but the lower cut - off is of little importance as the slope below 0.08  is very shallow . within the error bars , the three clusters all appear consistent with a `` standard '' kroupa - like imf . in particular , there is no evidence for an excess of high - mass stars in any of these clusters . we checked the curves in fig . [ fig : pmtol ] against the bruzual & charlot models , which are available for salpeter imf truncated at 0.1 @xmath44 and found very similar results , with differences at the level of 0.1 mag at most . it should be emphasized that this method does not constrain the exact shape of the imf . power - law imfs with a shallow slope , for example , would mimick the effect of a salpeter imf with a high - mass cut - off . it is becoming increasingly clear that `` massive '' star clusters can form in a wide variety of galaxies , and not just in mergers or otherwise disturbed galaxies . with the possible exception of some dwarf galaxies , the luminosity distributions of young star clusters generally appear to be power - laws . if cluster luminosities are sampled at random from a power - law distribution , the most luminous clusters will naturally be rare , but so far there is no evidence for a statistically significant upper cut - off . in other words , very luminous ( and massive ) clusters appear to form whenever clusters form in large numbers . this is illustrated by the fact that young star clusters with masses up to @xmath0 have been identified in the disks of several apparently normal , isolated spiral galaxies with rich cluster systems . these galaxies , such as ngc  5236 , ngc  6946 are characterised by high star formation rates , but these do not generally appear to be triggered by interactions with other galaxies . dynamical mass estimates are now available for a small number of these clusters , and the mass - to - light ratios are compatible with standard kroupa - type imfs . there is every reason to be optimistic that important clues to the formation of classical globular clusters may be obtained by studying their younger counterparts in the local universe . bica , e. , clari , j.  j. , dottori , h. , santos jr . , j.  f.  c. , piatti , a.  e. , 1996 , apj suppl . , 102 , 57 billett , o.  a. , hunter , d.  a. , & elmegreen , b.  g. , 2002 , aj , 123 , 1454 chandar , r. , bianchi , l. , & ford , h.  c. , 1999 , apjs , 122 , 431 chandar , r. , bianchi , l. , & ford , h.  c. , 1999 , a&a , 366 , 498 christian , c.  a. & schommer , r.  a. , 1982 , apjs , 49 , 405 christian , c.  a. & schommer , r.  a. , 1988 , aj , 95 , 704 dolphin , a.  e. , & kennicutt , r.  c. , 2000 , aj , 123 , 207 elson , r.  a.  w. , fall , s.  m. , & freeman , k.  c. , 1987 , apj , 323 , 54 fischer , p. , welch , d.  l. , ct , p. , et al . , 1992 , aj , 103 , 857 friel , e.  d. , 1995 , ara&a 33 , 381 girardi , l. , bressan , a. , bertelli , g. , & chiosi , c. , 2000 , a&as 141 , 371 gray , d.  f. , & toner , c.  g. , 1987 , apj , 322 , 360 harris , w.  e. , 1991 , ann . 29 , 543 jog , c.  j. , & solomon , p.  m. , 1992 , apj , 387 , 152 kennicutt , r.  c. & chu , y - h . , 1988 , aj , 95 , 720 kndlseder , j. , 2000 , a&a , 360 , 539 kroupa , p.  2002 , science , 295 , 82 lada , c.  j. , & lada , e.  a. , 2003 , ara&a 41 , 57 larsen , s.  s. , brodie , j.  p. , elmegreen , b.  g. , efremov , y.  n. , hodge , p.  w. , & richtler , t.  2001 , apj , 556 , 801 larsen , s.  s. , 2002 , aj , 124 , 1393 larsen , s.  s. , 2004 , a&a , in press larsen , s.  s. & richtler , t. , 1999 , a&a 345 , 59 larsen , s.  s. & richtler , t. , 2000 , a&a 354 , 836 larsen , s.  s. & richtler , t. , 2004 , a&a , in preparation salpeter , e.  e.  1955 , , 121 , 161 shapley , h. , 1930 , `` star clusters '' , mcgraw - hill ( new york ) smecker - hane , t.  a. , cole , a.  a. , gallagher iii , j.  s. , & stetson , p.  b. , 2002 , apj , 566 , 239 smith , l.  j. , gallagher , j.  s.  2001 , mnras , 326 , 1027 van den bergh , s. , 1999 , pasp , 111 , 1248 van den bergh , s. , & lafontaine , a. , 1984 , aj , 89 , 1822 whitmore , b.  c. , in : `` a decade of hubble space telescope science '' , eds . m.  livio , k.  noll , m.  stiavelli , uk : cambridge university press zepf , s.  e. , ashman , k.  m. , english , j. , freeman , k.  c. , sharples , r.  m. , 1999 , aj , 118 , 752
young star clusters with masses similar to those of classical old globular clusters are observed not only in starbursts , mergers or otherwise disturbed galaxies , but also in normal spiral galaxies . 
 some young clusters with masses as high as @xmath0 have been found in the disks of isolated spirals . 
 dynamical mass estimates are available for a few of these clusters and are consistent with kroupa - type imfs 
 . the luminosity ( and possibly mass- ) functions of young clusters are usually well approximated by power - laws . 
 thus , massive clusters at the tail of the distribution are naturally rare , but appear to be present whenever clusters form in large numbers . while bound star clusters may generally form with a higher efficiency in environments of high star formation rate , many of the apparent differences between clusters in starbursts and `` normal '' galaxies might be simply due to sampling effects . 
 it is a human habit to characterise those things with which we are most familiar as normal . 
 although large spiral galaxies are not the most common type of galaxy in the universe , we happen to live within one and many astronomers would probably tend to characterise the milky way as a fairly normal galaxy . 
 thus , at least for the purpose of this paper , `` normal '' galaxies mostly refer to non - interacting star forming disk galaxies . 
 our location within the milky way gives us a unique perspective from which we can study many of its properties in great detail , and it naturally provides a benchmark for comparison with other galaxies . 
 nevertheless , we should question whether it is justified to apply results obtained from studies of our own galaxy to other galaxies which may appear superficially similar to it . in the context of this workshop 
 , it is of particular relevance to ask how similar the cluster system in the milky way is to those in other galaxies . 
 an increasing amount of observational evidence is pointing to the conclusion that many spirals host `` young massive clusters '' ( ymcs ) or `` super star clusters '' similar to those observed in large numbers in starburst galaxies . 
 the definition of a ymc is rather vague and varies from one author to another , but the term generally seems to refer to young clusters that are more massive than the most massive open clusters in the milky way . 
 however , giving a meaningful definition of a massive cluster may eventually be as difficult as distinguishing galaxies that are normal from those that are not .