The Bcl-2 family of proteins plays a pivotal role in regulating cell life and death. Many of these proteins reside in the outer mitochondrial membrane, oriented towards the cytosol. Cytoprotective Bcl-2 family proteins such as Bcl-2 and Bcl-XL prevent mitochondrial permeability transition pore opening and release of apoptogenic proteins from mitochondria under many circumstances that would otherwise result in either apoptosis or necrosis. In contrast, some pro-apoptotic members of this family such as Bax can induce these destructive changes in mitochondria in both mammalian cells and when expressed exogenously in yeast. The mechanisms by which Bcl-2 family proteins control cell life and death remain elusive, but may include both the ability to form ion channels or pores in membranes and physical interactions with a variety of proteins implicated in apoptosis regulation.