This book presents a collection of contributions from the 'International School of Physics' "Enrico Fermi", held in Varenna, Italy in July 2008. This is the second school devoted to the astrophysics of galaxy clusters, and comes four years after the first; 'Background Microwave Radiation and Intra-cluster Cosmology'. The motivation for this second school was provided by the significant expansion in research into these, the largest gravitationally bound systems, and by the great interest in multifaceted observational, theoretical and numerical simulations of clusters. Topics were deal