  The thermal Sunyaev-Zel'dovich effect is an important probe of clusters of
galaxies, and has the attractive property of being proportional to the thermal
energy content of the intracluster medium. With the assistance of X-ray data,
the effect can be used to measure the number of hot electrons in clusters, and
thus measure cluster baryon contents. Cluster absolute distances and other
structural parameters can also be measured by combining thermal
Sunyaev-Zel'dovich, X-ray, and other data. This review presents an introduction
to the effect, shows some representative results, and sketches imminent
developments.
