PURPOSE OF REVIEW: Children and adults have two major types of adipocytes, which represent the predominant cells in white adipose tissue, which is involved in energy storage, and brown adipose tissue (BAT), which is responsible for thermogenesis and energy expenditure. This review discusses BAT physiology and evaluates the recent discoveries regarding its development, identification, and function. RECENT FINDINGS: Last year, multiple independent research teams using combined PET and computed tomography imaging, immunohistochemistry, and gene and protein expression have proven conclusively that adult humans have functional BAT. In parallel, basic studies defined BAT origins, its transcriptional regulation, and the role of hormones in BAT growth and activation. These methods have begun to be applied to children to understand pediatric BAT anatomy and physiology. SUMMARY: Adult humans have functional BAT, which plays a role in energy balance. BAT is more prevalent in children, suggesting an even greater physiological role than that seen in adults. Future studies will identify safe ways to quantify BAT mass and activity and which interventions might be used to increase BAT mass, thermogenesis, or both to treat obesity.