The correlation of neuronal activity is one of the main mechanisms underlying brain functions. Correlation occurs as a result of neural interaction and plays a role in the information processing of the brain. It also occurs in the changing brain during ontogeny and development as well as through learning and trauma-induced changes. This integrative approach investigates the presence and role of neural interaction in the vertebrate brain, both from the theoretical and experimental viewpoint. It relates much current neurophysiological work in visual, auditory, somatosensory as well as motoric systems and discusses plastic changes in the cerebellum, hippocampus and neocortex. All interpretations are discussed in light of several theories on plasticity and learning.