Macrophage activation syndrome (MAS) is a potentially life-threatening complication of rheumatic diseases such as systemic juvenile idiopathic arthritis (sJIA) and systemic lupus erythematosus. It is often considered a type of secondary hemophagocytic lymphohistiocytosis (HLH) and results from over-activation of T lymphocytes and macrophages leading to a "cytokine storm". Characteristic features are persistent fever, lymphadenopathy, hepatosplenomegaly, cytopenias (anemia, leucopenia, thrombocytopenia), raised C-reactive protein, falling erythrocyte sedimentation rate, hypofibrinogenemia, transaminitis, hypertriglyceridemia and extreme hyperferritinemia often associated with multi-organ impairment. Key to its management is early recognition of MAS which may be difficult due to similarity to systemic sepsis or flares of the underlying rheumatic disease. To aid with this process, criteria for the diagnosis of MAS in patients with sJIA derived by international consensus have been published. Although bone marrow biopsy showing hemophagocytosis is strongly supportive it is not essential for diagnosis. Together with appropriate supportive care, first-line treatment is high-dose intravenous corticosteroids with cyclosporin or intravenous immunoglobulin (IVIg) added if there is not initial response. Although etoposide is used by hematologists in treatment of HLH, there are concerns regarding organ toxicity and bone marrow suppression which weigh against its use in initial management of MAS. With increasing understanding of the pathogenesis of MAS, use of drugs targeting specific cytokines has been reported in case series. The relatively rapid effectiveness of anakinra, a recombinant IL-1 receptor antagonist, has been documented. Further studies of this and other biologic agents are required to identify the most effective and safest treatment option for refractory MAS.