The basic neuronal structure and circuitry of the cerebellum has been well known since Cajal's time. In recent years, however, a number of new neuronal connections and new immunohistochemically defined neuronal subtypes and functional cerebellar modules have been described. This new morphofunctional concept of the cerebellum seems to be in agreement with its newly assumed roles in learning and memory. In this new functional structure, a new cell (the brush cell, monopolar cell or monodendritic cell, Altman and Bayer, 1977; Mugnaini, 1994) specific to the cerebellar cortex and cochlear nucleus, could be of great importance. In all species studied, including man, this cell shows very particular morphology, immunohistochemical reactivity (against calretinin, some glutamate receptors and some neurofilament antibodies) and synaptic connections. The main afferents of these neurons are the mossy fibres, which form giant synaptic structures with them. The axons of monopolar cells end either in contact with extracerebellar neurons, or terminate intracortically at other brush cells (in the form of mossy fibres) or other cortical neurons of still-unknown morphology. In every animal species examined, these monopolar cells show different embryological development. No involution of them has been seen either in senility or in neurodegenerative disease.