Andisheh Mahdavi (UVIC) Title: The Canadian Cluster Comparison Project (CCCP) Clusters of galaxies are dominated by dark matter. We can see the gravitational effect of this dark material on the orbits of cluster members, the thermodynamics of the hot gas, and the shapes of galaxies behind the cluster. I will show that combining multiwavelength data for a single relaxed cluster can yield powerful constraints on its dark matter distribution and on the equation of state of the intracluster plasma. At the same time, as the bullet cluster teaches us, multiwavelength observations of merging clusters can yield significant and perhaps even more interesting constraints on dark matter properties. Both relaxed and merging clusters are well-represented in the Canadian Cluster Comparison Project, a CFHT survey of fifty massive nearby clusters. I will conclude by discussing an unusual, massive, X- ray bright region devoid of galaxies at the core of Abell 520, and will address its implications for our understanding of the nature of dark matter.