Unverricht–Lundborg disease (EPM1) is an autosomal recessively inherited neurodegenerative disorder and the most common single cause of progressive myoclonus epilepsy worldwide. Mutations in the gene encoding cystatin B (CSTB), a cysteine protease inhibitor, are responsible for the primary defect underlying EPM1. Here, progress toward understanding the molecular mechanisms in EPM1 is reviewed. We summarize the current knowledge about the CSTB gene and mutations as well as the cellular biology of the CSTB protein with emphasis on data emerging from analysis of EPM1 patients. We shed light on the disease mechanisms of EPM1 based on characterization of the CSTB-deficient mouse model.