This book focuses on hormones, and on how they are produced in very diverse regions of the body in humans and animals. But hormones can be found not only in vertebrates, but also in insects, shellfish, spiders, mollusks, even at the origin of metazoan diversification and exhibit the same pathways of synthesis. The book addresses the different classes of hormones: protein/peptides hormones, steroids and juvenile hormones and hormones like catecholamines, thyroid hormones and melatonin. It also discusses the types of hormone receptors, the majority of which are heptahelical G-protein coupled receptors or nuclear receptors. Particular attention is paid to the organs where hormones are created, with specifics on hormonal production and release, while a dedicated chapter details hormonal regulation from very simple to highly complex schemes. The remarkable kinetics of hormones production are also shown, before the book is rounded out by chapters on evolution in the endocrine system, the gen