The Natural History of Mitochondrial (MITO) Diseases (a longitudinal study observing the natural history of mitochondrial diseases) The goal of this observational study (non-randomised retrospective and prospective) is to fully characterise primary MITO disease; that includes both sexes/genders, over 18 years of age and healthy volunteers\]. The main question\[s\] it aims to answer is to: • better characterise MITO phenotypes (organ involvement, severity, progression) and collect biospecimens to create a biobank that can be used for future biomarker discovery to improve early diagnosis, prognostication and management of mitochondrial disease. The study will be a longitudinal, retrospective, prospective, observational study of participants (400) with confirmed MITO and relevant controls followed for up to 10 years. Data will be collected at regularly scheduled standard-of-care (SOC), 6 to 12 monthly appointments. The 100 control participants will therefore be comprised of (i) unaffected asymptomatic family members of MITO participants with no genetic risk; (ii) participants with non-MITO movement disorders that are not classified as MITO by their clinical presentation and genetic tests (for example Parkinson's disease) and/or (iii) age-matched healthy controls recruited from the NeuRA database of volunteers. Demographic data, medical history, biochemical, histological, genetic, social and other clinical SOC data will be collected. Additionally, seizure and migraine frequency in participants who experience these, will be collected and a quality-of-life questionnaire (SF-12v2), as part of the validated neurological assessment using the Newcastle Mitochondrial Disease Adult Scale (NMDAS).