The Griffith instability criterion for crack propagation is generalized to provide a criterion for time-dependent crack growth. Our procedure is similar to that of Griffith in its use of a global energy balance. The Griffith method is generalized by the inclusion of the rate-of-energy dissipation term in the balance law. This rate-of-dissipation term is derived to represent the behavior of viscoelastic materials; thus the final results are applicable primarily to polymeric behavior. Our final results are concise analytical expressions [Eqns. (50)] for crack velocity as a function of the creep properties of the material, the level of loading, and the energy or work content of the crack-generated new surface. These analytical solutions are quite general; the primary restriction occurs because they are derived under asymptotic conditions of high- and low-crack velocities. Finally, we compare the analytical predictions with experimental results for a polyurethane elastomer.