Obesity is a prevalent problem in modern society, which requires the upmost attention in the biomedical sciences. A leading cause of obesity related diseases is due to overeating, especially in industrialized countries. Leptin is the hormone that is secreted by fat cells responsible for communicating body nutritional status to the brain. Leptin interacts with other bodily systems such as the cognitive, digestive, neuronal, and endocrine systems. Leptin acts mainly on the Ob-Rb receptor in the arcuate nucleus of the hypothalamus and largely suppresses food intake and increases energy expenditure by activating Proopiomelanocortin and Cocaine- and amphetamine-regulated transcript (anorexigenic signals) neurons and by suppressing Neuropeptide Y and Agouti-related peptide (orexigenic signals) neurons, among other chemical signaling pathways. In both rodent and human studies, exogenous leptin administration resulted in elevated plasma leptin concentrations. When researchers tried to use leptin for weight reducing medical treatments in humans, the results show difficulty in establishing clinical efficacy. However, for diseases such as congenital leptin deficiency, obesity related leptin resistance, and lipodystrophy, medical treatments involving exogenous leptin have been relatively successful. The goal of this thesis is to give readers an understanding of leptin’s role in regulating appetite and the different leptin associated diseases. Leptin’s role is still continuing to be developed and more research is needed to fully utilize leptin for therapeutic benefit