Introduction

Finding extrasolar planets is good; learning something about their intrinsic properties is much better. Planets that are known only from their radial velocity signatures can be studied only in a limited sense: we can put a fairly reliable lower limit on their masses, and we can know the size and shape of their orbits. Transiting planets offer opportunities for more complete characterization: we can measure their radii with some precision, and in principle we can learn something of their temperature structure and of their atmospheres. For this reason, this review deals almost entirely with transiting planets. The plan of the paper is as follows: Section 3.2 introduces the basic geometrical and astrophysical ideas relating to transiting planets, and establishes the relationships among them.