The Sunyaev-Zel'dovich effect: an update

Much progress has been made on observing the Sunyaev-Zel'dovich effect(s) in clusters of galaxies, and recent theoretical work has placed the interpretation of the new data on a more secure footing. The deductions that can be made about the baryonic mass fractions of clusters and the value of the Hubble constant are reaching interesting levels of accuracy. In the present article I review the data, theory, and deductions and make some suggestions about where substantial progress is needed over the next few years by pointing out some of the pitfalls in the use of the effect.