Leptin is a versatile 16 kDa peptide hormone, with tertiary structure resembling that of members of the long-chain helical cytokine family. It is mainly produced by adipocytes in proportion to fat size stores, and was originally thought to act only as a satiety factor. However, the ubiquitous distribution of leptin receptors in almost all tissues underlies the pleiotropism of leptin. This review will focus on the link between leptin, a cytokine that is elevated in obese individuals, and cancer development through clarifying the intracellular signaling pathways of leptin. First, I discuss the biological functions of leptin and its signaling pathways. Then, I summarize the effects of leptin on different cancer types in experimental cellular and animal models. Next, I analyze the relationship between obesity and the presence of cancer or cancer risk in patients. Finally, leptin as a target for cancer treatment in the future and prevention will be discussed.