Ipercaloric diet and reduced physical activity have driven the rise in the prevalence of childhood obesity over a relatively short time interval. Family and twin studies have led to the conclusion that the strong predictive value of parental body mass index (BMI) mainly stems from genetic rather than environmental factors. Whereas the common polygenic obesity arises when an individual genetic make-up is susceptible to an environment that promotes energy consumption over energy expenditure, monogenic obesity, on the contrary, is the obesity associated with a single gene mutation, which is sufficient by itself to cause weight gain in a food abundant context. Genes involved in the leptin-melanocortin pathway are often mutated in these cases. The cumulative prevalence of monogenic obesity among children with severe obesity is about 5%. Recently, deletions in the region p11.2 of the chromosome 16 encompassing the gene SH2B1, which is involved in the leptin and insulin signaling, have been reported in about 0.5% of children with severe early-onset obesity. These patients show extreme hyperphagia, severe insulin resistance and, in some cases, mild developmental delay.