AN analysis of the results of nerve suture reveals great variations in the quality of the recovery even when repair is performed by skilled personnel operating under the most favourable conditions obtaining in the surrounding tissues. What is the explanation of these inconsistent results? Are the failures to be attributed to errors of technique when uniting the severed ends, to delay in repair or to morphological features in the nerve beyond the control of the surgeon? In this paper an attempt will be made to answer these questions by first denning the factors which influence the extent and quality of the recovery following nerve suture: in doing so, the policy will be to establish general principles and not to formulate details of technique. Until these factors have been identified, and the nature and the extent of the influence which each exerts precisely denned, attempts to interpret the incomplete and variable recoveries which follow nerve suture will be subject to considerable error. It will be shown that several factors always combine to influence the extent and quality of the recovery after nerve repair and that each of these is subject to a wide range of variation from individual to individual, nerve to nerve and, in respect of certain factors, from level to level along the same nerve. For this reason it is necessary, when evaluating a particular procedure or technique associated with repair (eg the relative merits of various suture materials and the usefulness of galvanic stimulation in therapy) or when assessing the influence on the end-result of a particular variable (eg sepsis and the interval elapsing between injury and repair), to consider each of the several variables that combine to affect the end-result and not to concentrate solely on the particular factor which is the central objective of the investigation. Only by adopting such a policy will it be possible to formulate principles which will make prognosis in these cases less uncertain and will it be possible to evaluate modifications of procedure and technique designed to improve recovery. The factors which influence the course of regeneration and the extent and quality of the recovery may operate central to the level of the injury, at the suture line or distal to it. Each will be considered in turn.