In this chapter, molecular weight (M), and molecular weight distribution (MWD), of polymers with emphasis on M and MWD of biopolymers, e.g., carbohydrate polymers, proteins, deoxyribonucleic acid, DNA, and ribonucleic acid, RNA, are reviewed. The M and MWD of biopolymers are compared with those of synthetic polymers. The following conclusions are drawn. (1) Unlike simple low molecular substances, most polymers do not have unique molecular weights. Practically, no polymer exists whose molecules are all strictly of the same size or have the same degree of polymerization. Thus, polymers are more or less heterogeneous with respect to their molecular weights. (2) The concept of average molecular weight is used for polymers. (3) Different numerical values for molecular weights of polymers have already been defined as average molecular weights (Mn, Mw, Mz, and Mv), depending on the methods by which they are measured. (4) The average values vary in the following order: Mn < Mv < Mw < Mz < Mz+1. The disparity between average molecular weights provides a measure of the degree of heterogeneity, i.e. dispersity, in the molecular weight distribution. (5) The constitution of a polymer as well as the MWD may be described either by a set of different average molecular weights, the ratios of two different types of average molecular weights, or by the distribution functions via graphical presentation and (6) Polysaccharides in a similar way to synthetic polymers are polydisperse polymers, whereas proteins, DNA, and RNA, are mostly monodisperse macromolecules.