The cerebellar granule cell, the most numerous neurons in the brain, forms the main excitatory neuron of the cerebellar cortical circuitry. Granule cells are synaptically connected with both mossy fibers and Golgi cells inside specialized structures called glomeruli, and thereby, they are subject to both feed-forward and feed-back inhibition. Their unique architecture with about four dendrites and a single axon ascending in the cerebellar cortex to bifurcate into two parallel fibers making synapses with Purkinje neurons has attracted numerous scientists. Recent advances show that they are much more than just relays of mossy fibers. They perform diverse and complex transformations in the spatiotemporal domain. This special issue highlights novel avenues in our understanding of the roles of this key neuronal population of the cerebellar cortex, ranging from developmental up to physiological and pathological points of view.