Apoptosis is a phenomenon fundamental to higher eukaryotes and essential to mechanisms controlling tissue homeostasis. Bcl-2 family proteins tightly control this cell death program by regulating the permeabilization of the mitochondrial outer membrane and, hence, the release of cytochrome c and other proapoptotic factors. Mitochondrial apoptosis-induced channel (MAC) is the mitochondrial apoptosis-induced channel and is responsible for cytochrome c release early in apoptosis. MAC activity is detected by patch clamping mitochondria at the time of cytochrome c release. The Bcl-2 family proteins regulate apoptosis by controlling the formation of MAC. Depending on cell type and apoptotic inducer, Bax and/or Bak are structural component(s) of MAC. Overexpression of the antiapoptotic protein Bcl-2 eliminates MAC activity. The focus of this review is a biophysical characterization of MAC activity and its regulation by Bcl-2 family proteins, and ends with some discussion of therapeutic targets.