Load-bearing biological materials such as bone, teeth, and nacre have acquired some interesting mechanical properties through evolution, one of which is the tolerance of cracklike flaws incurred during tissue function, growth, repair, and remodeling. While numerous studies in the literature have addressed flaw tolerance in elastic structures, so far there has been little investigation of this issue in time-dependent, viscoelastic systems, in spite of its importance to biological materials. In this paper, we investigate flaw tolerance in a visco-elastic strip under tension and derive the conditions under which a pre-existing center crack, irrespective of its size, will not grow before the material fails under uniform rupture. The analysis is based on the Griffith and cohesive zone models of crack growth in a visco-elastic material, taking into account the effects of the loading rate along with the fracture energy, Young's modulus, and theoretical strength of material. [http://dx.doi.org/10.1115/1.4007864] The article was published as: Chen L,Chen SH,Gao HJ. Flaw tolerance in a viscoelastic strip. JOURNAL OF APPLIED MECHANICS, TRANSACTIONS ASME, 2013, 80(4):041014/1-041014/10, doi: 10.1115/1.4007864