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                        X-RAY VISION:

 THE LARGE-SCALE DISTRIBUTION AND EVOLUTION OF GALAXY CLUSTERS


 Clusters of galaxies are prime tracers of the distribution and
 evolution of matter on the largest scales. Large, statistically
 well-defined cluster samples are needed for such studies but are
 hard to compile at optical wavelengths because of severe systematic
 biases that are difficult to quantify and thus often uncorrectable.

 I will present an overview of recent attempts aimed at overcoming
 the limitations of optical cluster searches by using X-ray
 selection. Specifically I will discuss and present results from
 three projects:

 - the first systematic X-ray search for clusters of galaxies behind
   the plane of the Milky Way that will allow us to identify and 
   account for the largest concentrations of mass affecting large
   scale flows in this optically obscured part of the local universe,

 - an X-ray search for distant (z>0.8) clusters of galaxies that 
   attempts to constrain cluster evolution at the highest redshifts
   and recently discovered the most massive galaxy cluster at z~0.9
   currently known,

 - a near-all-sky X-ray cluster survey that has already discovered
   ten times more very massive and distant clusters than any previous
   X-ray or optical survey, and that will allow us to probe with
   unprecedented accuracy the physical and cosmological framework
   of cluster formation.

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