Data are presented in this chapter for selecting materials to be used in the wide variety of applications of ultrasonic energy. These properties include moduli, Poisson’s ratio, densities, fatigue limits and ultimate stresses of the materials, sonic velocities, characteristic acoustic impedances, melting points, boiling points, electrical resistivities, thermal conductivities, compatibilities between materials, wear resistances, effects of oxide protective coatings, toughness or brittleness, resistances to acids and bases, corrosion resistance, protective atmospheres, chemical resistance, effects of stress risers, weldability, solderability, and effects of positions of welds and solders in an ultrasonically activated structure.