  I present an observational review of the five accretion-driven millisecond X-ray pulsars currently known, focusing on the results obtained with the Rossi X-ray Timing Explorer (RXTE) satellite. A prominent place in this review is given to the first such system discovered, SAX J1808.4-3658. Currently four outbursts have been detected from this source, three of which have been studied using RXTE. This makes this source the best studied example of all accretion-driven millisecond pulsars. Its October 2002 outburst is of particular interest because of the discovery of kilohertz quasi-periodic oscillations and burst oscillations during its thermonuclear X-ray bursts. The other four accreting millisecond pulsars were discovered within the last two years and only limited results have been published so far. A more extended review can be found at http://zon.wins.uva.nl/~rudy/admxp/ 
Observations of millisecond X-ray pulsars