Variation in the calpain 10 gene has recently been shown to be associated with type 2 diabetes by positional cloning. Since then, studies on calpain 10 have been started in correlation with diabetes and insulin-mediated signaling. In this review, the activation mechanism of calpain by calcium ions, which is essential to understand its physiological functions, is discussed on the basis of recent X-ray structural analyses. Further, special features of the structure of calpain 10 that differ from those of typical micro - or m-calpain used in most studies are summarized together with discussion of the physiological function of calpain with respect to type 2 diabetes.