This chapter reviews some data relevant to the physical chemistry of surfaces. The surface free energy 7 of a material body may be defined as the free energy required to create a plane free surface of unit area. The surrounding medium is thought of as a dilute phase that is not supposed to interact with the surface. Surface energy determinations require careful removal of surface active molecules from the bulk liquid, since these might get adsorbed on interfaces and substantially lower interfacial energies. A direct way of assaying solid-liquid interaction would be to measure the heat produced by mixing a liquid with a solid of known surface area, using a fine powder to ensure that this area is sufficient to yield measurable effects. The chapter describes some methods that were applied to biological surfaces with particular emphasis on contact angle and phase partition techniques.