This book is intended as a guide for anyone who is interested in animals and how their welfare can be assessed scientifically. It addresses the question of why, despite growing public interest in how animals are treated, it has proved so difficult to arrive at an agreed definition of what ‘animal welfare’ is and it then provides an answer. A definition of animal welfare as ‘health and animals having what they want’ can be easily understood by scientists and non-scientists alike, expresses in simple words what underlies many existing definitions and shows what evidence we need to collect to improve animal welfare in practice. Above all, it puts an animal’s own point of view at the heart of the assessment of its welfare. The book shows how ‘health and what animals want’ also helps us to make sense of the long and often confusing list of welfare measurements that are now in use, such as ‘stress’ and ‘feel-good hormones’, expressive sounds and gestures, natural behaviour, cognitive bias and stereotypies. Animal sentience (conscious feelings of pleasure and suffering) are discussed in the context of our current knowledge of human and animal consciousness. Finally, the book highlights some key ideas in the relationship between animal welfare science and animal ethics and shows how closely the well-being of humans and that of animals are linked together.