The circadian clock controls a large array of behavioral and physiological systems of fundamental importance to most organisms. Consequently, abnormal functioning of the clock results in severe dysfunctions and pathologies. Although epidemiological studies show a clear correlation between disruption of circadian rhythms and incidence of breast cancer, a molecular interpretation of how clock-related mechanisms may link to tumor development remains elusive. Here we speculate on the molecular pathways that may couple the circadian machinery to breast cancer.