Prof. Masao Ito contributed greatly to the elucidation of the structure and function of cerebellar neuronal circuits. He formulated a cerebellar motor learning theory and an internal model hypothesis and pursued how complex cerebellar functions resulted from the operation of neural circuits. In this article, we first provide a brief overview of the major cell types and the synaptic organization of cerebellar neural circuits. Then we introduce how mature cerebellar neural circuits are shaped through synapse formation and pruning during postnatal development. We also refer to the maturation of inhibitory neurons and glial cells during postnatal development.