This chapter reviews the basics of mitochondrial genetics and the clinical features and investigation of mitochondrial disease. Neurological presentations such as migraine, seizures, stroke-like episodes, myopathy, or neuropathy are not uncommon, but clinicians often only consider mitochondrial disease when these features occur in conjunction with other prompts such as deafness, diabetes, or a clear history of maternal transmission. Diabetes is commonly present in patients with mtDNA mutations, particularly those with the m.3243A>G mutation, and may be the presenting feature. Hepatic dysfunction and hemopoeitic stem cell failure (Pearson's syndrome) are uncommon features of mitochondrial disease but are seen more often in children than adults. The differential diagnosis for patients with mitochondrial disease is extensive owing to the varied clinical presentation. The major challenge in mitochondrial disorders, as in so many neurogenetic diseases, is to prevent the transmission and to develop effective therapies for those patients affected clinically.