Interhemispheric commissures link together selected regions of cerebral cortex in both brain hemispheres. Commissural fibres are more numerous in associative areas of the cortex than in sensory cortical fields. Corpus callosum consists of axons of pyramidal neurons situated in cortical layers II/III and V, which use glutamate as a neurotransmitter. Despite this, interhemispheric interactions can be both excitatory and inhibitory. Callosum enables interhemispheric transfer of information and learned associations, directing of attention, bilateral movement coordination and unitary perception of body surface. Activity of one hemisphere can influence plasticity of the other. However, published data provide very divergent experimental results. Damage in one hemisphere (for example due to stroke) impairs interhemispheric balance and often results in enhanced activation of areas of the intact hemisphere homotopic to the site of lesion, which can facilitate compensatory plastic changes. On the other hand, animal experiments show that plasticity in the contralesional hemisphere is decreased. This question is important especially for neurorehabilitation and requires further investigation.