Nitric oxide (NO) and its derivatives (reactive nitrogen species) have multiple effects on mitochondria that impact on cell physiology and cell death. Mitochondria may produce and consume NO and NO stimulates mitochondrial biogenesis, apparently via cGMP upregulation of transcriptional factors. NO inhibits mitochondrial respiration via: (A) an acute and reversible inhibition of cytochrome oxidase by NO in competition with O₂, and (B) irreversible inhibition of multiple sites by reactive nitrogen species. NO is a potent vasodilator (via cGMP), increasing O₂ and respiratory substrate supply to mitochondria. NO stimulates reactive oxygen and nitrogen species production from mitochondria via respiratory inhibition, reaction with ubiquinol and reaction with O₂ in the membrane. NO can induce apoptosis, mainly via oxidative stress. NO induces necrosis, mainly via energy depletion. Reactive nitrogen species activation of the mitochondrial permeability transition pore may cause apoptosis or necrosis. NO may protect against mitochondria-mediated cell death by multiple mechanisms.