AbstractSystemic lupus erythematosus (SLE) is a chronic, relapsing, inflammatory, often febrile multisystemic disorder, characterized by involvement of the skin, joints, visceral organs, and serosal membranes. Symptoms and manifestations vary widely over an unpredictable relapsing and remitting course.The presentation of SLE can range from mild forms to severe disease requiring hospitalization. Most commonly it manifests as a combination of constitutional symptoms, particularly fatigue and fever, with cutaneous, musculoskeletal, mild haematological, and serological involvement; however, when renal, haematological or central nervous system disease predominate it can be more severe, even life-threatening. There is a tendency for the disease pattern present at the time of onset to prevail during subsequent exacerbations.Investigating SLE depends to an extent on the presentation of the individual. However a number of haematological, biochemical and immunological investigations provide useful diagnostic information, either for the disease itself or in context of organ system involvement, and should be performed routinely.The presence of lupus nephritis should be considered in any lupus patient with impaired kidney function, proteinuria, hypertension, or an active urine sediment; the gold standard investigation in this context is a kidney biopsy. Glomerular immune complex deposition is the hallmark of lupus nephritis and underpins the International society of Nephrology/Renal Pathology Society classification of lupus nephritis.The diagnosis of SLE is based upon the presence of clinical and/or laboratory features and immunological markers that meet the various published diagnostic criteria. In 2012, lupus nephritis identified on kidney biopsy became an independent diagnostic criterion.This chapter goes through the clinical manifestations, investigations (including a detailed look at the kidney biopsy) and a review of the latest published diagnostic criteria.