  We highlight the importance of transit observations on understanding the
physics of planetary atmospheres and interiors. Transmission spectra and
emission spectra allow us to characterize this exotic atmospheres, which
possess TiO, VO, H2O, CO, Na, and K, as principal absorbers. We calculate
mass-radius relations for water-rock-iron and gas giant planets and examine
these relations in light of current and future transit observations. A brief
review is given of mechanisms that could lead to the large radii observed for
some transiting planets.
