Cellular circadian clocks represent a coordinated system of gene expression stimulated by both environmental and physiological cues that induces and maintains the rhythmicity of many metabolic processes. Circadian clocks confer the important benefit of anticipation of rhythmic environmental variation which serves to improve the health and survival of the organism. When disruption of these circadian patterns of gene expression occurs due to alterations in the physical (light/dark) and behavioral (feeding/sleeping) environments and/or due to genetic variation in the DNA sequence of clock component and clock regulated genes, negative health consequences can arise. One such consequence appears to be increased risk for cancer development. Circadian disruption has been associated with higher rates of tumorigenesis, faster tumor growth, and increased cancer severity in humans and animal models. Tumor formation is also associated with circadian disruption within the affected cells, and metabolic processes of the cancer host tend to lose their rhythmicity as the cancer becomes more severe. In addition, response to cancer treatment has been shown to have a time-dependent component in certain individuals. Knowledge of the type of circadian disruptions that induce or result from cancer can allow for temporally augmented treatments of cancer, ultimately making cancer treatments more effective and less harmful.