Clusters of galaxies are the largest organized structures in the Universe .
They are important cosmological probes , since they are large enough to contain a fair sample of the materials in the Universe , but small enough to have achieved dynamical equilibrium .
Clusters were first discovered as concentrations of hundreds of bright galaxies in a region about 3 megaparsecs ( 10 million light years ) across .
However , the dominant observed form of matter in clusters is hot , diffuse intergalactic gas .
This intracluster plasma has typical temperatures of T \sim 7 \times 10 ^ { 7 } K , and typical electron densities of n _ { e } \sim 10 ^ { -3 } cm ^ { -3 } .
This intracluster plasma mainly emits X-rays , and typical cluster X-ray luminosities are L _ { X } \sim 10 ^ { 43 } -10 ^ { 45 } erg/s .
The basic properties of and physical processes in the intracluster plasma will be reviewed .
Important observational constraints on plasma processes in these systems will be discussed .
Recent X-ray observations of clusters of galaxies with the orbiting Chandra X-ray Observatory will be highlighted .