The Purkinje cell (PC), central to the cerebellar cortical network, integrates excitatory and inhibitory synaptic inputs to generate the sole output of the cerebellar cortex. It receives two excitatory inputs: a single climbing fibre (cf) making multiple contacts on the dendritic tree, and approximately 200,000 parallel fibres (pf), the axons of granule cells, each of which make only one or two contacts. The cf generates an all-or-none discharge of the PC, while summation of many pf inputs appears necessary to generate a single spike. The excitability of the PC is regulated by inhibitory interneurons in the molecular layer. The contribution of individual neurons to the critical balance of excitation and inhibition remains unknown. I will record from pairs of synaptically connected neurons to determine the synaptic efficacy of pf and interneuron contacts, how these efficacies arise, and how excitatory and inhibitory inputs interact to generate the output of the PC.