  We present a synthesis of physical effects influencing the observed
lightcurve of an extrasolar giant planet (EGP) transiting its host star. The
synthesis includes a treatment of Rayleigh scattering, cloud scattering,
refraction, and molecular absorption of starlight in the EGP atmosphere. Of
these effects, molecular absorption dominates in determining the
transit-derived radius $R$. Using a generic model for the atmosphere of EGP
HD209458b, we perform a fit to the best available transit lightcurve data, and
infer that this planet has a radius at a pressure of 1 bar, $R_1$, equal to
94430 km. We predict that $R$ will be a function of wavelength of observation,
with a robust prediction of variations of $\pm 1$% at infrared wavelengths
where H$_2$O opacity in the high EGP atmosphere dominates.
