Neurophysiological measures can provide important information about the substrate of cognitive function in children, and most importantly, can give precise temporal information about ‘on-line’ brain function. These measures can be readily used in infants and children, and we present some examples of their application to understanding cognitive development. The most widely used of these techniques for developmental research is the method of event-related potentials (ERPs). In addition, the electroencephalogram (EEG) and the more rarely used invasive, intracranial investigations are also important to furthering our knowledge of how the brain–behaviour relations develop. The paper summarizes practical issues and presents some selected examples of experimental and clinical research.