A number of physical properties are relevant and important for characterizing adhesives, providing insights into the underlying behavior of the base polymer(s) as well as the effects that fillers, additives, and other factors may play in the liquid adhesive and bonded joints that may result. Characterizing these adhesive properties, interpreting the results, and interpreting the implications provides information that is vital assuring quality control, improving bond performance, assessing the relative merits of adhesive options for a given application, and understanding the polymer more completely. This chapter addresses several relevant physical properties of adhesives, including viscosity, density, and stress–strain behavior, quantities that are often thought to be intrinsic properties of a material. Methods of characterizing these properties are discussed, along with insights for interpreting these quantities and their implications for polymeric adhesive systems.