Hemophagocytic lymphohistiocytosis (HLH) is a rare life-threatening syndrome of uncontrolled immune activation. It was initially recognized in children, where it occurs primarily as an inherited syndrome related to homozygous null mutations in immune response genes involved in cytotoxic T cell and NK cell function. A minority of pediatric patients develop "secondary" HLH as a consequence of infection or autoimmune disease. In the last 10-15 years, secondary HLH has been increasingly recognized in adults, where it is frequently associated with lymphoid malignancy, infection, or autoimmune disease. This relatively recently recognized diagnosis and the treatment of adult HLH have been largely shaped by observations in pediatric patients. In this brief summary, we focus on the features that distinguish pediatric from adult HLH and discuss the challenges of diagnosis and treatment of this devastating disease.