Systemic lupus erythematosus (SLE or lupus) is a multisystem, autoimmune disease associated with the formation of autoantibodies that form pathological immune complexes and activate a number of inflammatory pathways. The disease is characterized by remissions and relapses (flares) that can present with a variety of clinical manifestations. The symptoms and signs may range from mild features that can be treated easily to organ and even life threatening manifestations requiring potent immunosuppression. This chapter will review the epidemiology and pathology of lupus, then the clinical features including differential diagnosis and investigation of adult patients with SLE. Finally the classification, diagnosis, monitoring and outcome of lupus patients will be discussed.