This book describes the essentials of a mathematical description of the dynamics of biochemical networks. It covers both deterministic and stochastic aspects of the dynamics. After providing a brief introduction to basic molecular biology, the book describes fundamentals of chemical kinetics.The chapter on signal transduction makes contact with ideas from feedback circuit analysis and signal processing. The chapter on switches and oscillators analyses in detail biological examples, both natural and synthetic. Excitable systems are introduced and contrasted with oscillators. The lastchapter deals with pattern formation and development and brings us to current questions of robustness of performance of developmental networks. The book provides brief introductions to some of the mathematical tools required in the main text and in a dedicated appendix. The emphasis, throughout, ison understanding of the essential dynamical aspects rather than just on recipes to build complex models.