After centuries of philosophical speculation about the existence of worlds beyond our solar system, the first hints of planetary mass objects orbiting other stars were reported in the late 1980s. A planetary system was discovered around a millisecond pulsar in 1992. Then, in 1995, based on precise radial velocity measurements of the host star, convincing evidence for the first exoplanet surrounding a main-sequence star was announced.

Two further exoplanets were known at the end of that year, and 34 at the end of the millennium. Since then, in just ten years, around 500 have been discovered through various methods, the 500th announced in December 2010 as this volume was going to press. A remarkable advance in understanding their physical, chemical and dynamical properties, and their formation and evolution, has kept pace with this discovery.

As the field has expanded, partly inspired by the vision of finding other Earths and perhaps other life, the stimulus provided to astronomical instrumentation has been equally profound. Amongst the technological advances are radial velocity accuracies unimagined twenty years ago, the accurate photometric monitoring of tens of millions of stars, alert systems which flag and focus attention of the world's planet hunters on gravitational microlensing events for a fleeting insight into invisible exoplanet systems, new coronagraphic techniques, a drive for extreme adaptive optics in unprecedented attempts to image alien worlds, and space missions devoted to their discovery and characterisation.