Compton scattering of the CMB by electrons in clusters of galaxies
imprints on the radiation a signature - the Sunyaev-Zeldovich effect -
whose spectral and spatial characteristics constitute indispensable
cosmological probes. The effect, essentially a redshift-independent
CMB image of a cluster, has been measured in many clusters by single
dish and interferometric arrays. Current and near-future measurement
capability, accurate theoretical description, and realistic modeling
of intracluster gas, have significantly advanced use of the effect as
a precise diagnostic tool to determine cluster and cosmological
parameters. I will describe the effect and the spatial CMB anisotropy
it induces, and will review the main results from current
measurements.