The dependence of stress‐strain curves on temperature and extension rate is first discussed qualitatively along with aspects of the fracture process. Considered thereafter are the strength and extensibility, measured over broad ranges of temperature and extension rate, of elastomers in several classes: (a) singlephase non‐crystallizable; (b) crystallizable, both unfilled and containing a reinforcing filler; and (c) block copolymers which contain plastic domains. It is shown that all single‐phase noncrystallizable elastomers lack toughness except under restricted test conditions and that toughness necessitates a dispersed phase. Data are presented on crystallizable rubber vulcanizates and on polyurethane and poly(urea‐urethane) block copolymers to show that plastic domains are highly effective sources of strength. The properties of these block copolymers are examined in detail, and their strengths are compared with those of other segmented copolymers and triblock elastomers. Factors responsible for strength are delineated. Copyright © 1977 Society of Plastics Engineers, Inc.