Review Talk on Clusters

In this talk, I will review some of the fundamental questions that are addressed with clusters of galaxies: What is the large-scale structure of the universe as traced by clusters of galaxies? How did clusters and large-scale structure form and evolve? What are the current cosmological constraints placed by clusters of galaxies - especially what is the amount, composition and distribution of matter in the universe? As we shall see, clusters provide a unique and powerful tool to address these questions they yield a robust measure of cosmological parameters including the mass-density of the universe and the amplitude of mass fluctuations.