  The Sunyaev-Zel'dovich effect causes a change in the apparent brightness of
the Cosmic Microwave Background Radiation towards a cluster of galaxies or any
other reservoir of hot plasma. Measurements of the effect provide distinctly
different information about cluster properties than X-ray imaging data, while
combining X-ray and Sunyaev-Zel'dovich effect data leads to new insights into
the structures of cluster atmospheres. The effect is redshift-independent, and
so provides a unique probe of the structure of the Universe on the largest
scales. The present review discusses the theory of the Sunyaev-Zel'dovich
effect and collects published results for many clusters, presents the overall
conclusions that may be drawn from the detections so far, and discusses the
prospects for future research on the Sunyaev-Zel'dovich effects.
