Book excerpt: Signalling within and between cells is an essential part of many biological processes, from the development of the body to the activity of our immune system. Cell Signalling, Third Edition, presents a carefully structured introduction to this intricate subject, introducing those conserved features that underlie many different extra- and intracellular signalling systems. Starting with an overview of cell signalling and highlighting its importance in many biological systems, the book goes on to explore the key components of extracellular and intracellular signalling mechanisms before examining how these components come together to create signalling pathways, which are so crucial to the survival of many living organisms. The text is enhanced by two-color artwork and 3-D protein models. A Companion Website provides resources for students and instructors.