Apoptosis is essential for the development, function and homeostasis of the immune system. Experiments with transgenic and gene knock-out mice have shown that defects in the control of apoptosis in the hematopoietic system can promote the development of autoimmunity or hematological malignancy. In contrast, excessive apoptosis of normally long-lived hemopoietic cells can lead to lymphopenia and immunodeficiency. In mammals, cell death in response to developmental cues and many cell stress signals is regulated by the opposing factions of the Bcl-2 family of proteins. In particular, the pro-apoptotic subgroup called BH3-only proteins, which includes Bim, is critical in the initiation of apoptosis in response to many death stimuli. Bim has been found to be an important regulator of the negative selection of B lymphocytes in the bone marrow and of T lymphocytes both in the thymus and the periphery. Mice lacking Bim accumulate self-reactive lymphocytes, develop autoantibodies and on certain genetic backgrounds succumb to SLE-like autoimmune disease. Abnormalities in Bim expression and the thymic deletion of auto-reactive lymphocytes have also been implicated as a component of the complex, polygenic predisposition to autoimmune diabetes seen in NOD mice. Bim is also an essential regulator of T lymphocyte apoptosis during the termination of an immune response. This chapter focuses on the role of Bim in the development and function of the immune system and its potential role in autoimmunity. Degenerative disorders due to increased apoptosis mediated by Bim are also discussed.