The hemophagocytic syndrome (HPS) is an uncommon, often misdiagnosed life-threatening disorder of immune regulation, characterized by a widespread proliferation and multisystemic infiltration of non-malignant histiocytes that undergo uncontrolled hemophagocytosis in bone marrow and/or reticulo-endothelial system. The HPS immune dysfunction consists in a low or absent cytotoxic T and natural killercell activity and in hyperactivation of T lymphocytes and macrophages, with consequent proinflammatory cytokine storm. Clinically, HPS is characterized by high fever, lymphadenopathies, hepato-splenomegaly, liver dysfunction, (pan)cytopenia, hyperferritinemia, hypertriglyceridemia, hypofibrinogenemia, as well as coagulopathy and neurological manifestations in many cases. A hereditary/primary variant (familial hemophagocytic lymphohistiocytosis) and an acquired/secondary form (reactive HPS) are known. This latter may occur in several disorders, including infections, immunodeficiency states, malignancies, lymphoproliferative and autoimmune diseases. Without treatment, HPS fatally has an unfavourable prognosis. Recently, outcome improvements have been reported, due to better therapeutic strategies. The clinical and pathological features of this syndrome are reviewed.