High prevalence of obesity in all of age categories is currently one of the biggest problem in medicine. Identification of etiology of obesity can individualise an approach to the patient and it is essential for choosing a target management and therapy. Beside the largest group with polygenic inheritance are clinically important also patients with "syndromic obesity", where obesity is only one of the signs and monogenic obesity, where obesity is the major clinical phenotype (patients with mutations in gene for leptin, leptine receptor, prohormone convertase 1, melanocortine receptor 4, brain-derived neurotropic factor and tyrosin kinase receptor B). The monogenic obesity includes 3-4% of all patients with obesity. This review article brings newest insight on genetics, clinical manifestation, diagnostics and therapy of these diseases.