Publisher Summary Osmotic pressure offers certain advantages in the study of polymers; the protein chemist often has a crucial advantage, that of dealing with the solutes of discrete sizes. Because of this, the practice of osmometry is simpler in several ways, and the molecular-weight values obtained for proteins are not attended with the uncertainties that arise from measurements on very heterodisperse solutes. The chapter points out certain applications and the more recent developments in technique that is, particularly relevant to the study of proteins. In work with proteins, the osmotic pressure of interest is the so-called colloid osmotic pressure or pressure difference that arises, because the protein component cannot diffuse through a membrane permeable to all other components. The procedures that have been used for measuring the osmotic pressure may be classified as belonging to either the static equilibrium method or to the dynamic equilibrium method.