Renal transplantation is the treatment of choice for end stage kidney disease. When allograft dysfunction is noted during follow up of patients, a biopsy is an extremely useful tool to diagnose cell mediated and antibody mediated rejection, and to rule out non-immunologic causes of graft malfunction, such as donor disease, ischemic/preservation injury, drug toxicity, infections, vascular thrombosis, post-transplant lymphoproliferative disease, and technical complications. In addition to its role in establishing the primary diagnosis, a biopsy can grade the severity of pathology present, help determine the most appropriate therapy, and provide information relevant to graft prognosis.