  I present an observational review of the five currently known
accretion-driven millisecond X-ray pulsars. A prominent place in this review is
given to SAX J1808.4-3658; it was the first such system discovered and
currently four outbursts have been observed from this source. This makes SAX
J1808.4-3658 the best studied example of the group. Its most recent outburst in
October 2002 is of particular interest because of the discovery of two
simultaneous kilohertz quasi-periodic oscillations and nearly coherent
oscillations during type-I X-ray bursts. This is the first time that such
phenomena are observed in a system for which the neutron star spin frequency is
exactly known. The other four systems were discovered within the last two years
and only limited results have been published. Since new exiting results are to
be expected in the future for all five sources, this review will only represent
a snap-shot of the current observational knowledge of accretion-driven
millisecond X-ray pulsars. A more extended and fully up-to-date review can be
found at http://zon.wins.uva.nl/~rudy/admxp/.
