The house mouse Mus musculus represents a valuable tool for the analysis and the understanding of the mammalian circadian oscillator. Forward and reverse genetics allowed the identification of clock components and the verification of their function within the circadian clockwork. In many cases unforeseen links were discovered between a particular circadian regulatory protein and various diseases or syndromes. Thus, this model system is not only perfectly suited to pinpoint the components of the mammalian circadian clock, but also to unravel metabolic, physiological, and pathological processes linked to the circadian timing system.