  The discovery of the first extra-solar planet surrounding a main-sequence
star was announced in 1995, based on very precise radial velocity (Doppler)
measurements. A total of 34 such planets were known by the end of March 2000,
and their numbers are growing steadily. The newly-discovered systems confirm
some of the features predicted by standard theories of star and planet
formation, but systems with massive planets having very small orbital radii and
large eccentricities are common and were generally unexpected. Other techniques
being used to search for planetary signatures include accurate measurement of
positional (astrometric) displacements, gravitational microlensing, and pulsar
timing, the latter resulting in the detection of the first planetary mass
bodies beyond our Solar System in 1992. The transit of a planet across the face
of the host star provides significant physical diagnostics, and the first such
detection was announced in 1999. Protoplanetary disks, which represent an
important evolutionary stage for understanding planet formation, are being
imaged from space. In contrast, direct imaging of extra-solar planets
represents an enormous challenge. Long-term efforts are directed towards
infrared space interferometry, the detection of Earth-mass planets, and
measurement of their spectral characteristics. Theoretical atmospheric models
provide predictions of planetary temperatures, radii, albedos, chemical
condensates, and spectral features as a function of mass, composition and
distance from the host star. Efforts to characterise planets occupying the
`habitable zone', in which liquid water may be present, and indicators of the
presence of life, are advancing quantitatively.
