The use of polymers as structural materials requires that these polymers retain sufficient strength even in the presence of preexisting microscopic cracks. These are tough and toughened polymers. This chapter discusses polymers with high toughness to enable their use as structural materials. The important microscopic deformation processes, such as crazing and shear yielding, that occur in the toughened polymeric materials are presented to understand the source of toughness. Crazing structure involves numerous oriented molecular chain bundles that are aligned together with many voids. The formation of molecular entanglements is an essential aspect of crazing. Although the formation of a single craze can induce crack initiation leading to brittle failure, the simultaneous formation of multiple crazes can provide a toughening mechanism that retards crack propagation. Unlike crazing, shear yielding does not involve void formation. Shear yielding is a common mode of plastic deformation in which the energy imparted to the material is absorbed by the deformation, and this phenomenon is an important toughening mechanism that inhibits crack propagation. Polymer blends will ideally form multiphase structures that optimize the sequence of these deformation events, resulting in the suppression of main crack propagation. Some of the important polymer blends based on engineering polymers, including both thermoplastics and thermosets, are described with a focus on phase structure control and toughening mechanisms.