Publisher Summary This chapter discusses event-related potentials and their diagnostic usefulness. There are now a very large number of published studies on the use of the visual evoked potential (VEP) in the diagnosis of multiple sclerosis. There is no doubt of the value of the method. What now concerns more is the difference in the success rate of different laboratories in detecting optic nerve plaques. It is a tribute to the robustness and reliability of the pattern response that one has managed to do as well, when one considers the enormous variation in the stimulus parameters used in the already published studies of the VEP in demyelinating disease. There are at least 3 different methods in general use for producing the checkerboard pattern reversal stimulus. The commonest still seems to be the slide projector and rotating mirror method. The spread of the latency values for both the normal and the pathological responses, represented by the magnitude of the standard deviation, allows one to arrive at an estimate of the sampling error when comparing one individual with another.