This chapter introduces the reader to the evidence-based philosophy of drug treatment, and then considers how it can be used in practice, including current sources of information to inform clinical practice, the grading of evidence, and the integration of evidence into decision making. Medicines are used throughout the National Health Service (NHS) to diagnose, cure or care for patients. There is a wealth of published literature available providing information on clinical efficacy and effectiveness of drug treatments. Primary research describes clinical trials whereby the 'gold standard' is considered to be the randomised controlled trial (RCT). Secondary research is a term used to describe reviews of primary research, conducted to standard research principles. Reviews can be anything from an individual specialist's overview of a subject, or a comprehensive literature review, to a systematic review. In relation to literature, the terms white and grey describe publication status and, by inference, quality.