This book deals with pain in companion animals and how it affects their welfare, health and, of course, behaviour. It describes the difficulties encountered, throughout history, to finally reach the conclusion that animals and humans share the same neurophysiological mechanisms to feel pain, and what their sensitive pathways are. It then goes deeper into the changes that pain can cause in the behaviour of dogs and cats and the tools available to the veterinary surgeon to control it. Good veterinary practice and professional ethics lead us to try to reduce pain in animals as much as possible. Having a book such as this one can help us achieve this objective.