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the idea to use _ initial state radiation _ to measure hadronic cross sections from the threshold up to the maximum energy of colliders with fixed energies @xmath1 , to reveal reaction mechanisms and to search for new mesonic states consists in exploiting the process @xmath2 to reduce the centre of mass energies of the colliding electrons and positrons and consequently the energy squared @xmath3 of the final state by emitting 1 or more photons . the method is particularly well suited for the modern meson factories like @xmath4 running at the @xmath5-resonance , _ pep - ii _ , _ kekb _ at the @xmath6-resonance with their high luminosities which compensate for the @xmath7 suppression of the emission of a photon . @xmath4 , _ pep - ii _ and _ kekb _ cover the energy regions up to 1.02 gev and up to 10.6 gev , respectively ( restricted for the latter actually up to 4 ... 5 gev if hard photons are detected ) . a big advantage of the _ isr _ method are the low point - to - point systematic errors of the hadronic energy spectra because the luminosity , the energy of the electrons and positrons and many of the various contributions to the detection efficiencies are determined once for the whole spectrum . as a consequence the overall normalization error is the same for all energies of the hadronic system . the term _ radiative return _ alternately used for _ isr _ refers to the appearance of pronounced resonances ( e.g. @xmath8 ) with energies below the collider energy ( see fig . [ fig : rsigma.eps ] , taken from ref . @xcite ) . reviews and updated results can be found in the proceedings of the international workshops in pisa ( 2003 ) @xcite , novosibirsk ( 2006 ) @xcite and the present ones in frascati ( 2008 ) . calculations of _ isr _ date back to the sixties - seventies of the @xmath9 century . for example photon emission for muon pair production in electron - positron collisions has been calculated in ref . @xcite , for the @xmath10-final state in ref . @xcite , resonances ( @xmath11 ) have been implemented in ref . @xcite , the excitation of @xmath12 and @xmath13 in ref . @xcite , and the possibility to determine the pion form factor was discussed in ref . the application of _ isr _ to the new high luminosity meson factories , originally to determine the hadronic contribution to vacuum polarization , more specifically the pion form factor , has materialized in the late nineties . it has started with calculations of _ isr _ for the colliders @xmath4 , _ pep - ii _ , _ kekb _ @xcite . see also refs . @xcite for calculations of radiative corrections for pion and kaon production below energies of 2 gev . an impressive example of _ isr _ is the _ radiative return _ to the region of the _ z_-resonance at _ lep 2 _ with collider energies around 200 gev @xcite , ( see fig . [ fig : rsigma.eps ] ) . -0.5 cm     as function of centre of mass energy @xmath1 , courtesy pdg ref . @xcite , title="fig:",width=264 ] -1.0 cm      _ isr _ became a powerful tool for the analysis of experiments at low and intermediate energies only with the development of _ eva - phokhara _ , a monte carlo generator which is user friendly , flexible and easy to implement into the software of the existing detectors @xcite . the generator was applied to an experiment to determine the cross section @xmath14 from the reaction threshold up to the maximum energy for the first time with the detector _ kloe _ at @xmath4 @xcite . the original motivation was and is still the determination of the hadronic contribution to the vacuum polarization from hadronic cross sections . the determination of the hadronic contribution to the vacuum polarization , which arises from the coupling of virtual photons to quark - antiquark - pairs @xmath16 , is possible by measuring the cross section of electron positron annihilation into hadrons with a real photon emitted in the initial state @xmath17 . it is of great importance for the interpretation of the precision measurement of the anomalous magnetic moment of the muon @xmath18 in @xmath19 @xcite and for the determination of the value of the running fine structure constant at the @xmath20 resonance @xmath21 , contributing to precision tests of the standard model of particle physics , see for details _ jegerlehner _ @xcite , also this conference @xcite . the _ novosibirsk _ groups @xmath22 and @xmath23 @xcite pioneered the measurements of hadronic cross sections by changing the collider energy ( _ energy scan _ ) . the _ isr _ method represents an alternate , independent and complementary way to determine those cross sections with different systematic errors . the hadronic vacuum polarization below 2.5 gev can be determined only experimentally because calculations within perturbative _ qcd _ are unrealistic , calculations on the lattice are not yet available with necessary accuracy , and calculations in the framework of chiral perturbation theory are restricted to values close to the @xmath27-threshold . with the maximum energy of 1.02 gev of @xmath4 almost 70 % of the hadronic correction can be determined due to the dominance of the @xmath28resonance . this correction was previously based on the measurement of the @xmath29 reaction of the _ novosibirsk _ groups @xcite by an energy scan . the _ kloe _ collaboration has carried out two analyses : in the so called small photon angle analysis the cross section @xmath30 has been measured between 0.6 and 1.0 gev with a statistical error of less than 0.2 % and a systematical error of 1.2 % . two pions have been detected in the drift chamber in the angular interval @xmath31 with excellent momentum resolution , disregarding the photon . however , an additional constraint for the missing photon angle @xmath32 has been required . as a consequence of this restricted phase space only the @xmath33 region between 0.3 and 1.0 @xmath34 is populated @xcite . the advantage of the small angle analysis is negligible background from resonant ( @xmath5 -)decays ( @xmath35 , @xmath36 and from final state radiation .    in the large photon angle analysis the cross section @xmath30 can be measured down to the @xmath27- threshold . in addition to the two pions also photons are detected in the angular interval @xmath37 with an energy of @xmath38 . this analysis is hindered by large resonant background and final state radiation which can be overcome by reducing the energy of @xmath39 below the @xmath5-resonance @xcite . the results of both analyses agree very well . the hadronic correction to the vacuum polarization for @xmath40 obtained in the small angle analysis using the data of 2001 ( @xmath41 in the interval @xmath42 is @xmath43 @xcite . the total systematic error of @xmath44 of @xmath45 includes an experimental systematic error of @xmath46 and a theoretical systematical error of @xmath46 taken in quadrature . an update ( mainly due to a revised _ bhabha _ cross section to determine the luminosity @xcite ) resulted in @xmath47 . using the data of 2002 ( @xmath48 ) the small angle analysis gave @xmath49 with a noticeably reduced statistical and systematic error . in the large angle analysis a value of @xmath50 is obtained for a restricted region @xmath51 covering mainly the @xmath25-resonance to be compared with the result of the small angle analysis of the 2002 data in the same energy interval of @xmath52 @xcite . the main problem of the large angle analysis is the large systematic ( model dependent ) error from the contribution of the @xmath53 from the radiative decay of the @xmath5 according to @xmath54 at low values of @xmath33 . the _ kloe isr _ data and the ( energy scan ) results from _ novosibirsk _ @xcite agree within the error bars ( fig . [ fig : kloecmd2 ] ) . -0.5 cm      soon after the application of _ eva - phokhara _ to _ kloe _ @xcite the _ babar _ collaboration also started the investigation of hadron final states with @xmath57 @xcite . in recent years a plethora of final states has been studied , starting with the reaction @xmath58 @xcite . while detecting a hard photon the upper energy for the hadron cross sections is limited to roughly 4.5 gev . final states with 3 , 4 , 5 , 6 charged and neutral pions , 2 pions and 2 kaons , 4 kaons , 4 pions and 2 kaons , with a @xmath5 and a @xmath59 , @xmath60 and 2 pions or 2 kaons , pions and @xmath61 , kaons and @xmath61 , but also baryonic final states with protons and antiprotons , @xmath62 and @xmath63 , @xmath62 and @xmath64 , @xmath65 and @xmath64 , @xmath66 and @xmath67-mesons , etc . have been investigated @xcite . in preparation are final states with 2 pions , 2 kaons . particularly important final states are those with 4 pions which contribute significantly to the muon anomalous magnetic moment and which were poorly known before the _ isr _ measurements @xcite . the _ babar _ results for the @xmath68- final states are not only consistent with previous ones but rather represent the best measurements for @xmath69 , are competitive for @xmath70 , are the best ones for @xmath71 and the only measurement for @xmath72 . detailed analyses allow the identification of intermediate states and consequently the study of the reaction mechanisms . for instance , in the case of the final state with 2 charged and 2 neutral pions @xmath73 the intermediate states @xmath74 and @xmath75 dominate , while also @xmath76 and @xmath77 have been seen . the final state with 5 pions proceeds to about @xmath78 via @xmath79 or @xmath80 and to about @xmath81 via @xmath82 , the rest being @xmath83 . for more details see @xcite . more recently also _ belle _ joined the _ isr _ programme with emphasis on final states containing mesons with hidden and open charm : @xmath60 and @xmath84 , @xmath66 and @xmath67 @xcite . a long standing problem is the nature of the low lying scalar mesons @xmath87 and their decay into pseudoscalars . _ kloe _ has studied the production of @xmath59 in the reaction @xmath88 @xcite . to separate the @xmath59 from the large irreducible background of initial and final state radiation the latter has been calculated by _ phokhara_. a large coupling of the @xmath5 to @xmath59 has been observed and no conflict found with the @xmath89 hypothesis , see also @xcite . many new highly excited states ( preliminarily denoted as _ x , y , z _ ) have been found in recent years . the first of them was discovered by _ babar _ in the reaction @xmath90 @xcite , a ( narrow ) state around 4260 mev with a width of 90 mev , later confirmed by _ belle _ via _ isr _ @xcite and by _ cleo _ in an direct energy scan @xcite . another state was detected at 2175 mev by _ babar _ in the reaction @xmath91 @xcite . _ belle _ found new states at 4080 , 4250 , 4360 , 4660 mev in the reactions @xmath92 and @xmath93 @xcite . the structure of basically all of these new states ( if they will survive ) is unknown so far . 4 quark states , e. g. a @xmath94[\bar{c } \bar{s}]$ ] state for @xmath95 , a @xmath96[\bar{s}\bar{s}]$ ] state for @xmath97 , hybrid and molecular structures are discussed , see also @xcite .      also baryonic final states with protons and antiprotons , @xmath62 and @xmath99 , @xmath62 and @xmath100 , @xmath65 and @xmath64 have been investigated in agreement with previous measurements , but with significantly improved statistics . the effective proton form factor shows some nontrivial structures at invariant @xmath101 masses of 2.25 and 3.0 gev , so far unexplained @xcite . being discussed since the sixties / seventies @xmath57 became a powerful tool for experimentalists only with the development of _ eva - phokhara _ by _ j. h. khn , h. czy , g. rodrigo _ and collaborators @xcite , a monte carlo generator developed over almost 10 years , while increasing its complexity , which is user friendly , flexible and easy to implement into the software of the existing detectors . the @xmath57 method was originally thought to be an alternative to energy scans to determine hadronic cross sections from the reaction threshold up to the maximum centre of mass energy of fixed energy meson @xmath102- factories in order to determine the hadronic contribution to the muon magnetic anomaly @xmath15 and to the running fine structure constant @xmath103 , and it was very successful in that respect . but being more than a _ poor man s _ alternative to scan energies it turned out to be an extremely useful tool to clarify reaction mechanisms and to reveal substructures ( intermediate states and their decay mechanisms ) leading to more and more hadronic final states , a plethora at @xmath104-factories . statistical errors have been dramatically reduced . systematic errors are under better and better control . finally it has opened a totally new and unexpected access to highly excited states ( with @xmath0 ) the structure of which is not yet known . h. czy , j. h. khn , eur . j. * c 18 * ( 2001 ) 497 ; g. rodrigo , a. gehrmann - 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the investigation of events with _ initial state radiation(isr ) _ and subsequent _ radiative return _ has become an impressively successful and guiding tool in low and intermediate energy hadron physics with electron positron colliders : it allows to measure hadronic cross sections and the ratio _ r _ from threshold up to the maximum energy of the colliders running at fixed energy , to clarify reaction mechanisms and reveal substructures ( intermediate states and their decay mechanisms ) and to search for new highly excited mesonic states with @xmath0 . while being discussed since the sixties - seventies _ isr _ became a powerful tool for experimentalists only with the development of _ eva - phokhara _ 
 @xcite , a monte carlo generator developed over almost 10 years , while increasing its complexity , which is user friendly , flexible and easy to implement into the software of existing detectors .