This chapter discusses the densities of amorphous and crystalline polymers. In general, the density of a polymer varies inversely with temperature but is much less sensitive to pressure. The introduction of fillers and plasticizers can alter the density and cause variations in density across a sample due to nonuniform distribution. Nevertheless, the average density is nearly invariant to small amounts of plasticizers. Amorphous polymers lack sufficient regularity in packing of the chains to produce the sharp X-ray diffraction pattern characteristic of highly crystalline polymers. The term crystalline polymer is actually a misnomer since no polymer is 100% crystalline, containing both crystalline domains and amorphous domains. The polymer chains are packed together more efficiently and tightly in the crystalline region than in the amorphous region, consequently the density of the crystalline region will typically be larger than that of the corresponding amorphous region. The relationship between the degree of crystallinity and the observed polymer density can be derived from two slightly different perspectives, depending on whether one assumes additivity of the crystalline and amorphous regions with respect to volume or to mass. The density of the crystalline region may be calculated from the dimensions of the unit cell, while the density of the amorphous region can be estimated by extrapolations from the melt density aided with a knowledge of the polymer's coefficient of thermal expansion. Experimental values of density are tabulated for some polymers designated by their common or trade name and chemical name.