Mitochondrial disorders are a group of heterogeneous, genetically inherited diseases that cause multiple symptoms in several organ systems. These disorders are usually progressive disorders affecting every age group, leading to severe morbidity and eventually mortality. Many children and adults with mitochondrial disorders exhibit intellectual and developmental disabilities, which can represent one of the initial complaints of the disease. This entry presents an overview of the mitochondria, its normal function, and the genetic defects resulting in mitochondrial disorders. The entry also discusses the complexity of mitochondrial disorders, their diagnosis, and management.