One of the most hotly contested issues in biology, or at least in mitochondrial biology, is determining which proteins can get into mitochondria. Having this kind of access essentially means a protein maintains a residence in one or more of several distinct mitochondrial compartments. These places include the matrix, the inner membrane, the intermembrane space, the outer membrane and the most nebulous listing of them all—an association with the outer membrane. Typically, it is this location that draws the biggest controversy.