BACKGROUND The role played by neurophysiological studies (NPS) in the diagnosis of diseases affecting neuromuscular transmission (NMT) is based on the study of the failure of muscle fibres to achieve a sufficient degree of depolarisation for the junction potential to reach the appropriate threshold and attain a muscular action potential. This totally or partially blocked impulse will give rise to different types of responses in neurophysiological tests. OBJECTIVE To analyse the different NPS as diagnostic methods in diseases that affect NMT. METHODS The article offers a review of the concept of the safety margin at the neuromuscular junction and a description of the most common neurophysiological techniques currently in use--repetitive stimulation, as well as conventional or single fibre electromyography (EMG) with voluntary activation or axonal electrical activation. The most frequent findings in diseases affecting NMT are also discussed. CONCLUSIONS NPS will be useful to confirm or reject the clinical diagnosis, to exclude other concomitant neuromuscular diseases, to establish whether the process is pre- or post-synaptic, to monitor the clinical course of the disease (when it is both natural or in response to the medical or surgical treatment) and also to enable the physician to determine the status of NMT in cases of clinical remission, as well as to detect subclinical disorders. Single fibre EMG studies are the most sensitive method of neurophysiological diagnosis when dealing with these diseases.