A major challenge of contemporary cell biology is to understand cell physiology from its underlying molecular regulatory networks. These networks are complex, containing many components interacting with one another through various positive and negative feedback loops. The dynamical consequences of these feedback loops are dauntingly complex, and it is not possible to understand them by intuitive arguments alone. Mathematical models are needed to describe the network precisely, to analyze their interactions rigorously and to provide accurate simulations that can be compared with experimental observations in quantitative details.