Hemophagocytic lymphohistiocytosis (HLH) is a potentially fatal hyperinflammatory syndrome with prolonged high fever, hepatosplenomegaly and characteristic laboratory findings. HLH may be inherited (primary) or may be secondary to any severe infection, malignancy or rheumatologic condition. The last several years have witnessed an explosion in our understanding of HLH. Of the inherited causes for which the underlying genetic cause is known, most involve abnormalities of proteins important in the exocytosis cytolytic pathway, whereby perforin and granzymes are delivered to a target cell to induce apoptosis. The exact mechanisms underlying this process remain unclear. However, when a known genetic defect is not present, the diagnosis of HLH is still made on a constellation of clinical features and good clinical judgment. Rapid diagnosis is crucial, as early therapy with immunosuppressive agents and/or proapoptotic chemotherapy can be life-saving. This article examines recent advances in our understanding of the pathophysiology, clinical features, diagnosis, etiology and treatment of HLH, as well as the challenges that lie ahead.