Obesity, a disease of both developed and developing countries, is spreading at an epidemic pace. According to the World Health Organization (WHO), obesity is defined as an increase or abnormal accumulation of body fat mass to the extent that an individual's health will be negatively affected. Overweight (i.e. body mass index [BMI] >25kg/m 2 ) is considered to be a significant risk factor for the development of many chronic diseases. Environmental, behavioural, genetic and biological factors interact to cause obesity, which can be viewed as a set of phenotypes that evolve over time in stages that need to be precisely measured. This article provides a clinical viewpoint on some biological processes that may explain some of the stages in the development of human obesity, its chronic maintenance and the occurrence of complications, with a focus on brain structures, genetics, the profound alterations in adipose tissue biology and the recent revival in thinking in terms of brown adipose tissue.