This article reviews the fundamental relationships between microstructure and mechanical properties for major classes of nonmetallic engineering materials: metals, ceramics and glasses, intermetallic compounds, polymers, and composites. It details the structures of inorganic crystalline solids, inorganic noncrystalline solids, and polymers. The article describes the various strengthening mechanisms of crystalline solids, namely, work hardening, solid-solution hardening, particle/precipitation hardening, and grain size hardening. Deformation and strengthening of composite materials, polymers, and glasses are reviewed. The article concludes with information on the two important aspects of the mechanical behavior of any class of engineering material: fatigue response and fracture resistance.