Human brown adipose tissue (BAT) can be activated to increase glucose uptake and energy expenditure, making it a potential target for treating obesity and metabolic disease. Data on the functional and anatomic characteristics of BAT are limited, however. In 20 healthy young men [12 lean, mean body mass index (BMI) 23.2 ± 1.9 kg/m²; 8 obese, BMI 34.8 ± 3.3 kg/m²] after 5 h of tolerable cold exposure, we measured BAT volume and activity by ¹⁸F-labeled fluorodeoxyglucose positron emission tomography/computerized tomography (PET/CT). Obese men had less activated BAT than lean men (mean, 130 vs. 334 mL) but more fat in BAT-containing depots (mean, 1,646 vs. 855 mL) with a wide range (0.1–71%) in the ratio of activated BAT to inactive fat between individuals. Six anatomic regions had activated BAT—cervical, supraclavicular, axillary, mediastinal, paraspinal, and abdominal—with 67 ± 20% of all activated BAT concentrated in a continuous fascial layer comprising the first three depots in the upper torso. These nonsubcutaneous fat depots amounted to 1.5% of total body mass (4.3% of total fat mass), and up to 90% of each depot could be activated BAT. The amount and activity of BAT was significantly influenced by region of interest selection methods, PET threshold criteria, and PET resolutions. The present study suggests that active BAT can be found in specific adipose depots in adult humans, but less than one-half of the fat in these depots is stimulated by acute cold exposure, demonstrating a previously underappreciated thermogenic potential.