Hemophagocytic lymphohistiocytosis (HLH) is a devastating disorder of uncontrolled immune activation characterized by clinical and laboratory evidence of extreme inflammation. This syndrome can be caused by genetic mutations affecting cytotoxic function (familial HLH) or be secondary to infectious, rheumatologic, malignant, or metabolic conditions (acquired HLH). Prompt recognition is paramount and, without early treatment, this disorder is frequently fatal. Although HLH is well described in the pediatric population, less is known about the appropriate work-up and treatment in adults. Here, we review the clinical characteristics, diagnosis, and treatment of HLH in adults.