Jeffrey Mogil, PhD, is the E. P. Taylor Professor of Pain Studies and the Canada Research Chair in the Genetics of Pain at McGill University in Montreal, Canada. He has made seminal contributions to the field of pain genetics, sex differences in pain and analgesia, and pain testing methods in the laboratory mouse. Here he discusses the definition of terms in the pain field, the evolutionary purpose of chronic pain, the behavioral assessment of pain in animals, and more.