The purpose of this study is to further enhance the existing knowledge regarding the side effects of belimumab when given with other lupus medicines to adults with active systemic lupus erythematosus (SLE). This study mainly focuses on collecting information on serious events that are not that common or may only be seen with long-term treatment. These events include death, serious infections and other infections of interest, cancers, serious mental health problems, including depression and suicide, and serious infusion and hypersensitivity reactions. This study is being done to help understand if treatment with belimumab increases the risk for these types of events. This study will also see if patients receiving belimumab with other lupus medicines can reduce their use of steroids, such as prednisone, over 1 year.