This is a Phase 3, multi-center, international, randomized, double-blind, placebo-controlled, 52-week study to evaluate the efficacy, safety and tolerability of belimumab administered subcutaneously (SC) (200 mg weekly) in adult subjects with active Systemic Lupus Erythematosus (SLE). Approximately 816 SLE subjects will be randomized, with a target of about 544 subjects receiving belimumab and 272 subjects receiving placebo. Subjects completing the 52-week double-blind period can enter a 6-month open-label extension in which all subjects receive belimumab 200 mg SC weekly.