Despite remarkable advances in pharmaceutical sciences, traditional and de novo drug discovery and development face significant barriers to obtaining marketing approval, including long timeframes, substantial financial investment, and high failure rates. Thus, a lot of effort is being made to bring out novel approaches to deal with the debilitating challenges facing the global pharmaceutical industry. Drug repurposing (or drug repositioning) represents one of the alternative strategies for traditional drug discovery, in which the process aims to seek new uses for already approved or investigational drugs beyond their original indication. Drug repurposing has been largely preferred owing to superior benefits related to reduced drug discovery timelines and costs. To date, a number of drugs have been successfully repurposed by providing effective solutions to various diseases that are especially hard to manage, making the drug repurposing a growing trend. This book's chapters offer comprehensive topics on drug repurposing and repurposed drugs.