This Section examines the forces governing the organization of molecular assemblies. We study a selection of DNA, protein, and lipid systems to understand general phenomena commonly described in term of molecular crowding, hydration, and binding. A primary goal is to characterize and codify measured electrostatic, van der Waals and hydration forces in such a way as to be used in computation and design of targeted measurements. Our undertaking is strengthened by a strong foundation in physical theory, in particular statistical mechanics as applied to liquid crystals and to complex fluids.