  The era of exoplanet characterization is upon us. For a subset of exoplanets
-- the transiting planets -- physical properties can be measured, including
mass, radius, and atmosphere characteristics. Indeed, measuring the atmospheres
of a further subset of transiting planets, the hot Jupiters, is now routine
with the Spitzer Space Telescope. The James Webb Space Telescope (JWST) will
continue Spitzer's legacy with its large mirror size and precise thermal
stability. JWST is poised for the significant achievement of identifying
habitable planets around bright M through G stars--rocky planets lacking
extensive gas envelopes, with water vapor and signs of chemical disequilibrium
in their atmospheres. Favorable transiting planet systems, are, however,
anticipated to be rare and their atmosphere observations will require tens to
hundreds of hours of JWST time per planet. We review what is known about the
physical characteristics of transiting planets, summarize lessons learned from
Spitzer high-contrast exoplanet measurements, and give several examples of
potential JWST observations.
