This document summarizes different types of anti-diabetic agents used to treat diabetes mellitus. It describes insulin and how it is synthesized, stored and secreted in the body. It also discusses oral anti-diabetic agents including insulin secretagogues, biguanides, thiazolidinediones, and alpha-glucosidase inhibitors; and how each group works to lower blood glucose levels. Complications from insulin therapy and factors affecting insulin absorption are also summarized.