Over the last two decades, the study of 'repetition effects' in behavioural and Event Related Potential (E.R.P) research has originated and added to theories of memory organisation. In this series of experiments, behavioural and E.R.P correlates of human and monkey visual memory were investigated, using the repetition effect as the main index of processing, and manipulating the semantic content of the experimental stimuli. The research has a large founding in established results from lexical studies. The use of pictorial material in this series of experiments extends these results to more general visual memory functions. E.R.P recordings were taken from two monkeys trained extensively to perform a matching-to-sample picture recognition task. The waveforms generated by novel instances of highly familiar pictures were compared to those elicited by repeats. In a further study with unfamiliar pictures the repetition of items had an effect on the evoked potentials for only one of the two subjects. Two further studies were made with one monkey viewing unfamiliar and familiar face pictures. In both these studies, an early potential emerged which was more positive in response to faces than to objects. In order to investigate the importance of the semantic content of stimulus items for memory processes, human E.R.Ps generated by novel and repeated presentations of 'meaningful' and 'meaningless' pictorial images were recorded. Repetition of the meaningful (but not meaningless) pictures attenuated the N400 component associated with the first presentation of a stimulus.