The fracture energy of moulded sheets of polyethylene has been found to depend strongly upon the thickness of the sheet, increasing linearly over the range 0.05 to 1.0 mm. This variation is attributed to a dependence of the volume of the plastic zone at the crack tip upont2, wheret is the torn thickness. By extrapolation, threshold values of fracture energy were determined at zero thickness. These represent the strength in the absence of large-scale plastic yielding. The fracture energies of both thin and thick sheets of high-density polyethylene (HDPE) were found to vary strongly with rate of tearing and test temperature, passing through maxima at particular rates and temperatures. This behaviour is attributed to corresponding changes in local ductility. A general correlation was found between the dependence of fracture energy upon tear rate and the dependence of loss modulusE″ upon the frequency ω of small oscillatory deformations. It is concluded that the work of fracture is mainly expended in local yielding at the tear tip, on the scale of single spherulites, about 4μm for HDPE. Annealing HDPE at 120° C reduced the fracture energy of thick sheets to about 10 kJ m−2 but did not greatly alter the threshold strength. In contrast, annealing sheets of LDPE had relatively little effect on the fracture energy at any thickness.