We first review the clinical presentation and current therapeutic approaches available for treating Unverricht-Lundborg disease (ULD), a progressive myoclonus epilepsy. Next, we describe the identification of disease causing mutations in the gene encoding cystatin B (CSTB). A Cstb-deficient mouse model, which recapitulates the key features of ULD including myoclonic seizures, ataxia, and neuronal loss, was generated to shed light on the mechanisms contributing to disease pathophysiology. Studies with this model have elucidated the diverse biological roles for Cstb from functioning as a protease inhibitor, to regulating glial activation, oxidative stress, serotonergic neurotransmission, and hyperexcitability. These findings set the stage for future studies that may open avenues to improved therapeutic approaches.