Serotonin (5-hydroxytryptamine, 5-HT) was first described as the 'serum tonic factor', and was therefore named serotonin. Serotonin is widely present in the brain, including in the cerebellum, which is richly innervated by serotonergic fibres. A variety of serotonin receptors mediate the effects of serotonin in the cerebellum. These serotonin receptors all have their own specialized role, but with some similar main effects. It is through the temporally and spatially restricted expression of these different serotonin receptors in the cerebellum that such a widely expressed neurotransmitter as serotonin can exert very specific functions. These functions include regulation of neuronalactivity, synaptic transmission and cerebellar development.