Publisher Summary This chapter reviews that in general charged molecules that are hydrophilic at neutral pH do not readily penetrate into the mitochondrial matrix space. The inner mitochondrial membrane acts as a permeability barrier to the free diffusion of metabolic anions and cations between the cytosolic and mitochondrial spaces of the cell. In contrast, molecules of low molecular weight appear to pass readily through the outer mitochondrial membrane and become distributed in a space at least equal to that occupied by sucrose. A passive permeability of the mitochondrial inner membrane is exhibited by certain anions of high lipid solubility such as SCN–, which become distributed across the membrane in accordance with the magnitude of an existing membrane potential. Monovalent ions of weak acids and bases, such as CH3COO– and NH+4, also enter the mitochondrial matrix volume passively, the transported species through the membrane lipid bilayer being the undissociated acid or base. Transport is thus a function of the concentration of undissociated acid in the aqueous phase and its solubility and diffusion coefficient in the lipid phase of the mitochondrial membrane. The chapter also discusses three types of translocators: (1) electroneutral, (2) electroneutral proton compensated, and (3) electrogenic.