This book treats the interfacial forces between biopolymers, cells and other polar and apolar polymers, particles and surfaces. In the attractive mode, some of these interfacial interactions have become familiar to many as “hydrophobic interactions”. Certain of the repulsive aspects of the forces have been perceived earlier as “hydration pressure”. The quantitative importance of various manifestations of these interactions has been widely accepted, but their precise nature had only been partly elucidated until fairly recently.