Discussing the definition of pressure sensitivity and characterization of pressure-sensitive behavior, Volume 1 of the Handbook of Pressure-Sensitive Adhesives and Products presents the underlying theory behind the main criteria of pressure sensitivity, including Dahlquist criterion, free volume theory, and fibrillation theory, and the pressure-sensitive performance characteristics defined by tack, peel resistance, and shear resistance. It describes the chemical and macromolecular basis of pressure sensitivity as determined by molecular mobility and its parameters and molecular structure and its regulation. The book also addresses the physical and mechanical basis of pressure sensitivity along with the mechanical properties of pressure-sensitive adhesives and products that correlate to their adhesive, converting, and end-use performance characteristics.