It is often stated that the theoretical limit for the velocity of crack propagation in materials relates to the free surface Rayleigh wave velocity or other wave related criteria [Roberts DK, Wells AA. Engineering 1954;171:820; Field JE. Contemporary Physics, 1983;24(5):439; Schardin H. In: Averbach BL, Fellbeck DJK, Hahn GT, Thomas DA, editors. Fracture, chap. 16, New York: Wiley, 1959; Anderson OL. In: Averbach BL, Fellbeck DJK, Hahn GT, Thomas DA, editors. Fracture, chap. 17, New York: Wiley, 1959.] However, the limiting crack velocity in many materials is less than this theoretical limit and considerably so for tough polymers. Using experimental fracture data obtained from a range of tough polymers, and a computer model in which toughness, stiffness and other material parameters can be independently varied, relationships have been identified between limiting crack velocity and toughness properties of polymers. (C) 2000 Elsevier Science Ltd. All rights reserved.