Cerebellar damage disrupts motor coordination, making movements variable and inaccurate. Rehabilitation is the primary treatment for cerebellar motor disorders (i.e. 'ataxia') since there are no proven pharmacological treatments. However, the effectiveness of current rehabilitation methods is limited. This necessitates both a better understanding of cerebellar function and better rehabilitative treatments following cerebellar damage. This proposal focuses on a new role for the cerebellum in proprioceptive localization. We are specifically studying if the cerebellum helps people to sense their hand position in space after active (self-generated) movement versus passive (externally-driven) movement, how this relates to their motor problems, and if non- invasive cerebellar stimulation can help them. Aim 1 quantifies proprioceptive localization deficits in people with cerebellar damage, to determine whether the cerebellum plays a role in this ability following active movement. Preliminary results indicate that cerebellar subjects have deficits in hand localization after active movements, but not passive movements. This suggests that cerebellar predictions of self-generated movement might help proprioception. Aim 2 tests whether we can temporarily reproduce cerebellar-like deficits in healthy people using inhibitory, non-invasive cerebellar stimulation (i.e. cathodal transcranial direct current stimulation or tDCS). This would be powerful evidence that cerebellum-dependent predictions of movement are a key mechanism for disrupting active proprioception. Aim 3 investigates whether excitatory non-invasive cerebellar stimulation (anodal tDCS) in people with cerebellar damage is a potential rehabilitation strategy. It will determine whether anodal tDCS enhances the residual cerebellum's processing ability, and thus improves proprioceptive localization and movement control. The research proposed here will improve understanding of the role of the cerebellum in proprioception, advance knowledge of how proprioceptive deficits relate to motor control, and investigate whether modulating cerebellar excitability with non-invasive brain stimulation is a potential rehabilitatio strategy for people with cerebellar damage.