This chapter describes and evaluates the current body of research on viscosity perception, and identifies areas where further research is needed. To better interpret the sensory findings, a brief description of the physical properties that define solution viscosity is presented first. In general, studies on viscosity perception have either a descriptive or more fundamental aim. Studies designed to be descriptive compare the similarities and differences between physical and perceptual measures of viscosity; those with a more fundamental aim seek to use the relationship between physical and sensory measures of viscosity in order to understand how humans perceive viscosity. Both types of studies require considerable care in the interpretation of sensory results. Investigators have taken advantage of the different shear stress/shear rate functions of non-Newtonian thickeners in an attempt to identify the range of shear rates and stresses that develop when human subjects evaluate solution viscosity.