While the evidence is compelling that the endogenously produced molecule, melatonin, inhibits the initiation, progression and metastasis of breast cancer, the mechanisms proposed to explain how melatonin achieves these actions are varied and numerous.Melatonin may modulate breast cancer via mechanisms that involve cell membrane receptors, nuclear receptors or cytosolic binding sites or its actions may be receptor independent, i.e., by regulating the redox status of the cell via its powerful free radical scavenging capabilities.