Personalized medicine refers to the use of biomarkers to help define appropriate therapies for patients with a defined disease, predict their prognosis, and monitor their response to therapy. When these biomarkers are used to identify patients who are either likely to respond to a specific drug, or who are at risk for experiencing adverse reactions to a specific pharmacological treatment, they are often referred to as Companion Diagnostics (CoDx). CoDx are co-developed by diagnostic companies and the pharmaceutical industry and are subject to regulatory approval. Following implementation in clinical laboratories, they play an important role in patient care. This short course will offer useful practical information about how companion diagnostics are developed, how they move through the regulatory process, and how they pass from clinical trials into routine use.