  We highlight how guaranteed time observations (GTOs) and early release
science (ERS) will advance understanding of exoplanet atmospheres and provide a
glimpse into what transiting exoplanet science will be done with JWST during
its first year of operations. These observations of 27 transiting planets will
deliver significant insights into the compositions, chemistry, clouds, and
thermal profiles of warm-to-hot gas-dominated planets well beyond what we have
learned from HST, Spitzer, and other observatories to date. These data and
insights will in turn inform our understanding of planet formation, atmospheric
transport and climate, and relationships between various properties. Some
insight will likely be gained into rocky planet atmospheres as well. JWST will
be the most important mission for characterizing exoplanet atmospheres in the
2020s, and this should be considered in assessing exoplanet science for the
2020s and 2030s and future facilities.
