  Using a full frequency-dependent atmosphere code that can incorporate
irradiation by a central primary star, we calculate self-consistent boundary
conditions for the evolution of the radius of the transiting planet HD 209458b.
Using a well-tested extrasolar giant planet evolutionary code, we then
calculate the behavior of this planet's radius with age. The measured radius is
in fact a transit radius that resides high in HD 209458b's inflated atmosphere.
Using our derived atmospheric and interior structures, we find that irradiation
plus the proper interpretation of the transit radius can yield a theoretical
radius that is within the measured error bars. We conclude that if HD 209458b's
true transit radius is at the lower end of the measured range, an extra source
of core heating power is not necessary to explain the transit observations.
