A theory for the tensile strength of amorphous plastics is presented. It extends a previous theory so as to include the effects of imperfections and chain relaxations. The result obtained shows that the time taken for a plastic to break,tc, under constant stress,F, is given byF=−Bln(tc)+C,whereBandCare molecular constants. The experimental values found for these constants are shown to be consistent with their molecular interpretation. This extended theory shows that the effect of chain relaxations is the major time dependent factor as far as strength is concerned.