In the 1990s circadian rhythm researchers made enormous progress in identifying the components and operations within the responsible mechanism in various species using the tools of molecular biology. In the past decade it has proven essential to supplement these with the tools of systems biology both to identify additional components but especially to understand how the mechanism can generate circadian phenomena. This has proven especially important since research has shown that individual neurons in the mammalian mechanism are highly variable and that the way they are organized in networks is crucial to generating regular circadian behavior.