The organ most frequently affected in mitochondrial disorders, particularly respiratory chain diseases (RCDs), in addition to the skeletal muscle, is the central nervous system (CNS). CNS manifestations of RCDs comprise stroke-like episodes, epilepsy, migraine, ataxia, spasticity, movement disorders, psychiatric disorders, cognitive decline, or even dementia (mitochondrial dementia). So far mitochondrial dementia has been reported in MELAS, MERRF, LHON, CPEO, KSS, MNGIE, NARP, Leigh syndrome, and Alpers-Huttenlocher disease. Mitochondrial dementia not only results from mutations in the mitochondrial genome but also from mutations in nuclear genes, such as POLG, thymidine kinase 2, or DDP1. Often mitochondrial dementia starts with specific cognitive deficits, particularly in visual construction, attention, abstraction, or flexibility but without a general intellectual deterioration. Cognitive impairment in RCDs is diagnosed upon neuropsychological testing, imaging studies, such as MRI, PET, or MR-spectroscopy, CSF-investigations, or electroencephalography. Therapy of mitochondrial dementia relies on symptomatic measures. Only single patients profit from cholinesterase inhibitors or memantine, antioxidants, vitamins, coenzyme-Q, or other substitutes. Overall, mitochondrial dementia is an important differential of dementias and should be considered in patients with multi-system disease.