  I present an overview of our current observational knowledge of the six 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, three of which
have been studied in detail using the Rossi X-ray Timing Explorer satellite.
This makes SAX J1808.4-3658 the best studied example of an accretion-driven
millisecond pulsar. 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 (and so far only) time that such phenomena are observed in a
system for which the neutron star spin frequency is exactly known. The other
five systems were discovered within the last three years (with IGR J00291+5934
only discovered in December 2004) and only limited results have been published.
