Since phase changes in crystalline materials alter the microstructure, they also have a strong influence on the physical and mechanical properties, including the complex elastic modulus (dynamic elastic modulus and damping). This paper reviews some of the influences of phase changes on the damping of crystalline materials. Specific examples of the effects are presented for crystallographic order-disorder processes (for example, in 75Cu-25Au), partial melting of constituents in dental alloys (mercury-tin-silver) and in leaded alpha brass (copper-zinc-lead), magnetic transformations near the Curie point (in iron, nickel, and 75Ni-25Co), different microstructural phases in an alloy (uranium-titanium), precipitation processes (in manganese-copper alloys), and ferroelectric and dielectric processes (in potassium bromide-potassium cyanide). Most of these examples are taken from results obtained with the PUCOT (piezoelectric ultrasonic composite oscillator technique) operated in the longitudinal vibrational mode at frequencies of 40 to 150 kHz, temperatures in the range of 300 to 1115 K, and strain amplitudes in the range of 10-7 to 10-4. The prospects of controlling the level of damping in alloys through the control of the phase changes are outlined.