The study of microdeformation in polymers helps provide a link between macroscopic toughness and microstructural and molecular parameters, and hence guidelines for the development of new, improved materials Investigation of micronecking in thin films of glassy polymers by transmission electron microscopy has been instrumental in the development of entanglement-based models for polymer deformation. These have subsequently invoked to account for a variety of phenomena, from crack resistance in homogeneous amorphous glassy polymers to interfacial strength in reaction bonded heterogeneous semicrystalline interfaces. The present discussion of these models is primarily illustrated with the results of TEM investigations of deformation in thin films and thin sections taken from deformed bulk samples. Parallels are drawn between the behaviour of amorphous glassy polymers and semicrystalline polymers, and rubber modified systems.