This timely volume presents specially written articles by world experts at an international conference at the Space Telescope Science Institute. The goal of the meeting was to assemble physicists and astronomers working on all aspects of dark matter and theories of gravity. Topics covered include Nucleosynthesis, Hot Gas in Clusters, MACHOs, WIMPs, Rotation Curves, Gravitational Lensing Neutrinos, Large Scale Flows, Dwarf Spheroidals, Cosmological Parameters from Supernovae, the Cosmic Microwave Background, the Cosmological Constant, and Theories of Gravity.