The work focuses on the role of a recently discovered protein with an endocrine function: leptin. It turned out to be a key factor for the regulation of food intake in several animal species, as highlighted in many research activities. Experimentations on mice and rats and also studies involved morbidly obese patients confirmed this very important role. In particular, first, the structure and gene expression of leptin are discussed, and then its functions are examined, focusing on metabolic pathways involved in food behaviour and intake regulation. The various effects of leptin deficiency are analyzed, including the ones caused by synthesis defects of the protein itself and of its receptor as well. They include an insatiable hunger, which leads to a dramatic food intake increase not related to a general metabolism enhancement. As a result, the adipose tissue increases as well, leading the patients to morbid obesity. The reasons of some typical phenotypic aspects are explained in detail: they actually depend on the kind of mutation occurring, and some of them may appear bizarre and not related to obesity. After this analysis, conclusive considerations based also on recent scientific works will be made. © 2017 Nova Science Publishers, Inc.