Human brown adipose tissue has been acknowledged in newborns and children but in adults the first printed publications are from the beginning of 20th century. Further evidence of the existence of adult brown fat was published throughout the century but only very recently the functionality of active brown adipose tissue in vivo in adulthood was confirmed. This was contributed mainly by advanced imaging technology, namely hybrid positron emission tomography (PET) and computed tomography (CT), being able to combine functional and anatomical imaging data. Functionality is most commonly measured with glucose analog, 18F-fluoro-2-deoxy-d-glucose (FDG) but other tracers for other functions than glucose uptake have been introduced as well. Growing body of evidence has increased the knowledge of the role of brown adipose tissue in human metabolism and energy expenditure, providing a promising option for the management of body weight balance and disturbed glucose and lipid metabolism.