Twenty participants from Denmark, Finland, France, Germany, Italy, Norway, Sweden, The Netherlands, the UK, and the USA met to discuss the treatment of mitochondrial disorders in children and adults. The group included adult and paediatric neurologists, metabolic paediatricians, an ophthalmologist, an endocrinologist, and other clinical scientists. The aim of the workshop was to define the extent of the clinical problem (the epidemiology of mitochondrial disease), discuss methods of assessing disease severity that might be used to assess treatment response, discuss the current best practice for the treatment of mitochondrial disease, and explore novel treatment strategies. During the workshop, the participants discussed their own clinical experience of the management of mitochondrial disease and by the end of the workshop, the participants produced two documents which are included in this report: