Loss of Ca²⁺ homeostasis, often in the form of cytoplasmic increases, leads to cell injury. Depending upon cell type and the intensity of Ca²⁺ toxicity, the ensuing pathology can be reversible or irreversible. Although multiple destructive processes are activated by Ca²⁺, lethal outcomes are determined largely by Ca²⁺-induced mitochondrial permeability transition. This form of damage is primarily dependent upon mitochondrial Ca²⁺ accumulation, which is regulated by the mitochondrial membrane potential. Retention of the mitochondrial membrane potential during Ca²⁺ increases favors mitochondrial Ca²⁺ uptake and overload, resulting in mitochondrial permeability transition and cell death. In contrast, dissipation of mitochondrial membrane potential reduces mitochondrial Ca²⁺ uptake, retards mitochondrial permeability transition, and delays death, even in cells with large Ca²⁺ increases. The rates of mitochondrial membrane potential dissipation and mitochondrial Ca²⁺ uptake may determine cellular sensitivity to Ca²⁺ toxicity under pathological conditions, including ischemic injury.