Thin spherulitic firms of polyethylene (PE), made by casting from xylene solution, were deformed on copper grids and their deformation microstructure studied using optical and transmission electron microscopy. A range of molecular weights, branch amounts and types, and thermal histories was used to study the influence of sample microstructure on the deformation behaviour. The spherulite boundaries were the weakest regions of spherulitic structures in all types of PE. Higher strains were generally required to deform the interiors of spherulites, which deformed by a different mechanism. The toughness of the films was increased by increasing the tie-molecule density and disentanglement resistance, to strengthen spherulite boundaries. Three methods for increasing the toughness were demonstrated and were : (1) decreasing the crystallization time in order to increase the tie-molecule density, (2) increasing the molecular weight in order to increase the tie-molecule density, and (3) incorporating short-chain branches on the main chains in order to increase the tie-molecule density and increase the disentanglement resistance. Incorporation of short-chain branches was shown to be the most effective way of strengthening spherulitic structures, with the toughest films being those made of branched PE that had been crystallized rapid ly. The branch length and spherulite size were found to be unimportant.