This document discusses orphan drugs, which treat rare diseases. It defines orphan drugs and outlines criteria used in the US and EU to designate drugs for rare diseases. Rare diseases affect fewer than 200,000 people in the US and 10,000 in the EU. The Orphan Drug Act of 1983 in the US provided incentives for orphan drug development. Similar laws exist in other countries. Developing orphan drugs is challenging due to low patient populations and high costs. Examples of orphan drugs and their manufacturers are provided. The process for obtaining orphan drug designation is also summarized.