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Extensive recent work on the Sunyaev-Zeldovich (S-Z) effect reflects major progress in observational capabilities of interferometric arrays, the improved quality of multi-frequency measurements with upcoming ground- based and stratospheric bolometer arrays, and the intense theoretical and experimental work on the small scale structure of the cosmic microwave background (CMB) radiation. I briefly describe the effect and discuss its significance as a major cosmological probe. Recent results for the gas mass fraction in clusters and the Hubble constant (largely from measurements with the BIMA and OVRO interferometric arrays) are discussed. Also reviewed are results from the first determination of the CMB temperature at the redshifts of two clusters (from measurements with the MITO and SuZIE experiments), and recent work on the CMB anisotropy due to the S-Z effect.

Scattering of the cosmic microwave background (CMB) radiation by hot gas in clusters of galaxies produces a unique spectral signature — the Sunyaev—Zeldovich (S—Z) effect — that constitutes an important cosmological probe. The effect has been sensitively measured in more than 40 clusters, mostly with ground-based interferometric arrays operating at low microwave frequencies. These measurements have already yielded important information on cluster masses, and the Hubble constant. The scientific yield will be greatly increased when spectral and high-resolution spatial measurements of the effect in a large sample of nearby clusters will be made with stratospheric telescopes equipped with bolometric arrays. I review the current status of observational and theoretical S—Z work, and describe the main goals and challenges of using the effect as a more precise probe of cluster properties and cosmological parameters.

Comptonization of the cosmic microwave background (CMB) radiation by hot gas in clusters of galaxies-the Sunyaev-Zeldovich (S-Z) effect- is of great astrophysical and cosmological significance. In recent years observations of the effect have improved tremendously; high signal-to-noise images of the effect (at low microwave frequencies) can now be obtained by ground-based interferometric arrays. In the near future, high frequency measurements of the effect will be made with bolomateric arrays during long duration balloon flights. Towards the end of the decade the PLANCK satellite will extensive S-Z surveys over a wide frequency range. Along with the improved observational capabilities, the theoretical description of the effect and its more precise use as a probe have been considerably advanced. I review the current status of theoretical and observational work on the effect, and the main results from its use as a cosmological probe.

The theoretical framework of Comptonization of the spectrum of the Cosmic Microwave Background radiation by intracluster (the Sunyaev-Zeldovich effect) and intergalactic gas is briefly reviewed. The Planckian spectrum of this radiation field may had been Comptonized by the recombining matter in the early Universe, as seems to be supported by recent measurements of the submillimeter excess. This affects the traditional treatment of the Sunyaev-Zeldovich effect in ways which we point out. The effect is a unique probe which will eventually be applied to obtain extremely valuable astrophysical and cosmological information; we discuss some of the likely uses of the effect.

We have analyzed existing data on colours and Tully-Fisher distances of a sample of spiral galaxies in the Virgo cluster. The data seem to indicate a significant correlation between the colour-excesses and distances of these galaxies. This correlation may be due to extinction by intracluster (IC) dust. However, due to uncertainties in the observational data, and the limited size of the sample, such an interpretation may not yet be construed as an unequivocal evidence for dust in the Virgo cluster of galaxies.

The full extent of beam dilution in measurements of the Zeldovich-Sunyaev (Z-S) effect can be appreciated when it is realized that the parameters characterizing intracluster gas density and temperature profiles assume values within a wide range. We discuss the possible role of beam dilution in previous measurements of the effect. We also estimate the contribution of hypothetical intra-supercluster hot gas to the Z-S signal along a line of sight to a cluster which is a memeber of a supercluster.

Dark matter in clusters can be either more or less centrally concentrated than galaxies, depending on whether it was once bound to galaxies in halos. If the distributions of galaxies and dark matter are represented by similar profiles, but with different core radii, then the radial variation of the velocity dispersion in the Coma cluster and the degree of efficiency of gravitational drag on galaxies are somewhat more consistent with the dark matter being more centrally concentrated than galaxies.