  When extrasolar planets are observed to transit their parent stars, we are
granted unprecedented access to their physical properties. It is only for
transiting planets that we are permitted direct estimates of the planetary
masses and radii, which provide the fundamental constraints on models of their
physical structure. In particular, precise determination of the radius may
indicate the presence (or absence) of a core of solid material, which in turn
would speak to the canonical formation model of gas accretion onto a core of
ice and rock embedded in a protoplanetary disk. Furthermore, the radii of
planets in close proximity to their stars are affected by tidal effects and the
intense stellar radiation. As a result, some of these "hot Jupiters" are
significantly larger than Jupiter in radius. Precision follow-up studies of
such objects (notably with the space-based platforms of the Hubble and Spitzer
Space Telescopes) have enabled direct observation of their transmission spectra
and emitted radiation. These data provide the first observational constraints
on atmospheric models of these extrasolar gas giants, and permit a direct
comparison with the gas giants of the Solar system. Despite significant
observational challenges, numerous transit surveys and quick-look radial
velocity surveys are active, and promise to deliver an ever-increasing number
of these precious objects. The detection of transits of short-period
Neptune-sized objects, whose existence was recently uncovered by the
radial-velocity surveys, is eagerly anticipated. Ultra-precise photometry
enabled by upcoming space missions offers the prospect of the first detection
of an extrasolar Earth-like planet in the habitable zone of its parent star,
just in time for Protostars and Planets VI.
