**Abstract**

In this chapter, we describe the possible link between gut microbiota, melatonin, and breast cancer disease. It is widely described that changes in melatonin production due to circadian disruption is one of the causes of breast cancer. In addition, recently it is described that dysbiosis caused by changes in the gut microbiota composition could be as well constitute an important factor to induce breast cancer. The dysbiosis process, in turn, induces the stimulation of kynurenine pathway, leading to reduced circulating melatonin levels. Therefore, in this chapter we deep into the relationship between circadian disruption, dysbiosis, and breast cancer disease. This constitutes an important step in the therapeutic approach and prevention of this pathology.

**Keywords:** melatonin, breast cancer, gut microbiota, circadian disruption, dysbiosis, estrogens, estrobolome, anticancer therapies
