Detailed molecular dynamics simulations performed to study the nature of lipid raft domains that appear in model membranes are reviewed in this paper. The described simulations were performed on hydrated bilayers containing binary mixtures of cholesterol with phospholipids and also on ternary mixtures containing cholesterol, a phospholipid with a high main transition temperature T(m), and a phospholipid with a low transition temperature T(m). These simulations provide qualitative and semi-quantitative information about cholesterol-lipid interactions and also a testing ground for major assumptions made to explain the nature of lipid rafts in model membranes.