The strength and toughness of polymers depend on stress-induced dissipative and disruptive processes exemplified by viscoelastic and plastic deformations, cavitation, and crack growth. High strength necessitates molecular immobility; toughness results from mobility and the dissipation of stored elastic energy. These concepts are discussed in terms of the dissipative and disruptive processes that occur under stress in various types of elastomeric and plastic materials. The role of structure at the molecular and supermolecular levels is considered along with that of elastic constraints which, by altering the stress state, modify strength and toughness. To have high toughness and strength, a material must contain a dispersed phase. When effective, the dispersed phase acts to increase the dissipation of energy, often through controlled structural break-down, and to impede the development of large cracks. © 1970, Walter de Gruyter. All rights reserved.