Three crystalline polymers; high- and low-density polyethylene andtrans-polyisoprene; were prepared as sheets with varying degrees of molecular orientation. Measurements of the energyG c required to propagate a tear along and across the direction of orientation are described. The tear energy was found to depend linearly upon the thickness,t, of the sheet, in all cases. This dependence in attributed to plastic yielding at the crack tip in a zone having a cross-sectional area proportionalt 2. The dependence on thickness was greater for highly oriented sheets torn across the orientation direction. This is attributed to a greater extent of plastic yielding in the orientation direction, as indicated by model experiments. By exhapolation to zero thickness of sheet, values were obtained of threshold tear energy in the absence of large-scale plastic yielding. However, these values were still relatively large (1 to 50 kJ m−2), and they were directly proportional to the extension ratio imposed across the tear direction at the time of crystallization. Thus, for tearing parallel to the orientation direction, both the threshold strength and the additional contribution torn plastic yielding were small, whereas for tearing across the orientation direction both were large, Values of the effective diameter of the tear tip for unoriontod materials were deduced torn the threshold strengths. They were about 35μm for HDPE and about 90μm for LDPE and TPI; about five times the average spherulite diameter.