Medication errors are believed to be a leading cause of avoidable harm to patients around the world, with an estimated cost of US$42 billion per year, worldwide. It has been estimated that 5% of all patients who are admitted to a hospital experience a medication error, and that an average hospital will have one medication error every 22.7 hours or every 19.7 admissions. Human error is inevitable, but many of these errors actually reflect failures in the design and resourcing of the system within which medications are administered to patients, and some reflect violations of safe medication practices. Medication errors are the most common of all medical errors and pose a tremendous emotional and physical cost to patients and economic burden to our health system. Clearly medication error is a major source of risk and adverse events for our patients. This chapter presents a roadmap for the rest of the book, and definitions that will be used throughout. We also discuss how medication error is measured, the methods by which it is studied, the techniques used to capture incident reports, and some of the metrics and statistics to report these errors and incidents.