Written for students and researchers in systems biology, the second edition of this best-selling textbook continues to offer a clear presentation of design principles that govern the structure and behavior of biological networks, highlighting simple, recurring circuit elements that make up the network. Rigorously classroom-tested, it contains new additions as well as corrections and revisions for better flow. This edition includes four new chapters: perfect robustness, scaling in development, noise and variability in biological circuits, and evolution of modularity. It also doubles the number of exercises and adds an appendix.