The Moonwalker (Mwk) mouse is a model of dominantly inherited cerebellar ataxia. The ataxic phenotype is caused by a gain-of-function mutation in the gene encoding the cation-permeable transient receptor potential channel TRPC3. Mwk mice display early-onset motor coordination defects and a loss of balance. TRPC3 is highly expressed in cerebellar Purkinje cells and type II unipolar brush cells that both degenerate in the adult Mwk mouse. In addition, Purkinje cells harboring the Mwk mutation do not develop normally and show reduced dendritic arborization and synaptogenesis. The Mwk mutation affects TRPC3 channel gating and results in altered excitability of Purkinje cells. Downstream effects include altered calcium homeostasis and changes in lipid metabolism. An increasing number of human spinocerebellar ataxias are associated with impairments of mGluR1-TRPC3 signaling, making the Mwk mouse a relevant disease model.