Evolution of the mass function of clusters of galaxies is a sensitive measure of the cosmological parameters, especially Omega. I will discuss some recent results on cluster evolution from the CfA-IfA-ROSAT survey (160 deg2) which found more than 40 X-ray bright clusters at z>0.4. I will discuss how to use the cluster baryon masses as a proxy for the total mass function and argue that this is a better approach than using temperatures or X-ray luminosities. An application of this method to the 160 deg2 sample indicates a strong evolution of the cluster mass function since z=0.5. The observed evolution defines a narrow band in the Omega-Lambda plane which intersects with the cosmological constraints from CMB and SNIa near Omega=0.3 and Lambda=0.7.

X-ray observations also is an excellent tool for the study of the physical processes in the intracluster medium. I will present the Chandra observations of two clusters, A3667 and Coma.

In Abell 3667, we observe the brightness edge which turns out to be a boundary of a large cool gas cloud moving through the hot ambient gas. The cloud velocity is accurately measured from the ratio of the interior and exterior pressures. We find M=1+-0.2 or v=1400+-300 km/s and observe two additional features expected for a slightly supersonic velocity --- a bow shock and gas compression at the leading edge of the cloud.

In the Coma cluster, we the interstellar gas with a temperature of 1-2 keV is confined by the hot intergalactic medium of the Coma cluster into compact clouds. The physical state of the gas in these clouds appears to be determined by a delicate balance between the radiative cooling and suppressed (by a factor of 30--100) heat conduction through the interface between these clouds and the hot cluster gas.

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