Brown fat is present and metabolically active in adults, and is involved in physiologic and pathologic processes. Brown fat activation can seriously interfere with correct [(18)F]fluorodeoxyglucose-PET image interpretation if not recognized, although this can be overcome through identification of typical uptake patterns, pharmacological and nonpharmacological interventions to reduce brown fat uptake, and hybrid PET/computed tomography and PET/magnetic resonance imaging technologies. Advanced quantitative PET imaging techniques may be used to quantify total brown fat volume and metabolic activity for assessment prior to and following various interventions, likely contributing to further understanding of brown fat.