Beginning with an introduction to sentience, the book investigates why we are so interested in sentience, when humans became sentient, and how it has changed over time. The book defines aspects of sentience such as consciousness, memory and emotions, and discusses brain complexity in detail. Examining sentience from a developmental perspective, it analyzes when sentience appears and uses evidence from a range of studies investigating embryos, fetuses and young animals to form an enlightening overview of the subject.