The circadian clock is an endogenous time keeping system shared by most organisms. In mammals, a master pacemaker in the hypothalamus orchestrates temporal alignment of behavior and physiology by transmitting daily signals to multiple clocks in peripheral tissues. Disruption of this communication has a profound affect on human health and has been linked to diverse pathogenic conditions, including cancer. At the center of the molecular circadian machinery is a set of clock genes, generating rhythmic oscillations on a cellular level. In the past several years, research from different fields has revealed the complexity and ubiquitous nature of circadian regulation, uncovering intriguing associations between clock components and cellular pathways implicated in tumorigenesis. In this review, we discuss the emerging role of circadian genes in hematological and hormone-related malignancies. These new insights suggest that manipulating circadian biology as a way to fight cancer, as well as, other life threatening diseases is within the realm of possibility.