Nerves have a structure of considerable complexity with features of special relevance to nerve injury and nerve regeneration. These include variations in the cross-sectional areas devoted to fascicular and epineurial tissue, the fascicular redistribution and mixing of different branch fibers brought about by fascicular plexuses, and the numbers of nerve fibers representing individual branches. The elasticity and tensile strength of nerve trunks and their capacity to resist traction deformation reside in the fascicular tissue, while the epineurium provides a protective cushion against compression. The microstructure of nerve trunks provides the basis for a classification of nerve injuries into five degrees of severity with partial and mixed types--each with a clearly defined pathology and distinguishing clinical features. Following a transection injury, changes occur in the severed axons, endoneurial tubes, fasciculi, and nerve trunk. The type of injury and the nature of these changes determine the outcome of axon regeneration.