Mitochondrial diseases affect one in 2,000 individuals; they can present at any age and they can manifest in any organ. How defects in mitochondria can cause such a diverse range of human diseases remains poorly understood. Insight into this diversity is emerging from recent research that investigated defects in mitochondrial protein synthesis and mitochondrial DNA maintenance, which showed that many cell-specific stress responses are induced in response to mitochondrial dysfunction. Studying the molecular regulation of these stress responses might increase our understanding of the pathogenesis and variability of human mitochondrial diseases.