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The principal baryonic component of clusters of galaxies is diffuse gas held in hydrostatic equilibrium in the gravitational potential of the cluster. This gas is hot (107 - 108 K), relatively dense (10-4 - 10-2 atom cm-3), and enriched with heavy elements (e.g., Fe of 0.3 Solar abundance). This combination results in significant X-ray emission through thermal bremsstrahlung radiation. Detailed X-ray observations of clusters can provide us with accurate total mass measurements, clues to the merger history of clusters, and a chemical record of the supernova ejecta that polluted the intracluster medium during the formation of the stars in the member galaxies.

The X-ray emission from clusters can also be exploited to select clusters irrespective of their member galaxies. While the optical selection of clusters is well established and understood, there are potential problems with projection and the imperfect scaling of the galaxy population to total cluster mass that make independent selection methods attractive.

There are four key considerations for any X-ray survey:

There are a number of cluster properties that can be used to constrain the nature and evolution of clusters.

Each of these requires either dedicated pointed observations or a survey drawn from the brightest serendipitous detections in pointed observations. The former is a relatively slow process requiring time allocation committees to put substantial resources into programs to observe "complete" samples. The latter is very slow given the area covered by sufficiently deep X-ray observations.

For the purposes of this review I will define "low redshift" as z < 0.5 and treat any paper presenting any new X-ray detection of cluster as a "survey."

In my talk I used the yardstick of exponential growth to judge progress in known numbers of X-ray emitting clusters which I modestly named "Edge's Law." This holds that for every decade of X-ray astronomy the number of clusters detected increases by an order of magnitude. I would like to stress that this was a narrative device and not a serious bid for future surveys in itself. That said, the rapid progress in cluster research in the past decades does require us to stand back and assess it as part of a larger picture.

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