Childhood obesity is an important public health issue worldwide. Urbanization, sedentary lifestyle and change in food habits are the chief reasons behind this pandemic. In a small proportion of children, obesity is the result of endocrine, syndromic or monogenic causes. The present paper summarizes the methods, definitions and cut-offs for identification of obesity in children. We have briefly reviewed the various techniques used for estimation of body fat in children and the cut-offs for defining obesity based on body fat percentage, and the reference curves based on body mass index and waist circumference. The etiology of obesity in children, including individual behaviors, macro- and micro-environmental influences, and endocrine causes have been discussed, and an approach to etiological assessment of obese children has been presented. Special emphasis has been laid on clinical pointers that suggest the presence of syndromic, endocrine or monogenic forms of obesity, such as, short stature, dysmorphism, neurocognitive impairment and early age at onset.