Circadian rhythms are daily oscillations in metabolic, psychological and physiological processes in all living organisms. Deregulation of these rhythms is associated with a number of pathological conditions including depression, diabetes, metabolic syndrome and cancer. One such condition is breast cancer. The global impact of breast cancer is large and growing. The risk of breast cancer is 5 times higher in developed countries than in underdeveloped once. It is clear that the modern lifestyle influences the disorders of the circadian system and thus the occurrence of breast cancer. This thesis, comprehensively analyses current literature data on the involvement of circadian clock components in breast cancer development. Emphasis is on the analysis of clock gene expression and methylation in tumor tissue as well as on the influence of melatonin as a major circulating rhythm output factor on the onset and progression of breast cancer. Major findings from human studies indicate that circadian gene expression is deregulated in breast cancer. Decreased expression of PER, CRY and CLOCK genes has been observed, while TIMELLESS gene expression is often elevated, which is associated with poorer disease prognosis. Disruption of the expression of these genes might be related to their methylation. Further investigation on the role of circadian genes in the pathogenesis of breast cancer is required to complement current knowledge and for better understanding of the molecular mechanisms, underlying processes leading to carcinogenesis.