The discovery of leptin has led to an understanding of the regulation of energy expenditure and food intake in a number of vertebrates. Leptin is also implicated in diverse physiological functions. In poultry, leptin cDNA was first cloned from chicken to assess the roles of leptin; however, this cDNA has not been confirmed by a number of researchers nor genomic DNA corresponding to the chicken leptin has been found. This controversy is one of the most important issues to be elucidated in avian endocrinology. Avian leptin receptor cDNA was first identified in chicken in 2000 and studies suggest that the receptor could mediate the leptin signal to exhibit its physiological functions. In this review, we summarize the recent findings in avian leptin research, including physiological roles and signal transduction.