We have around 3000 mitochondria in each and every cell in our bodies. They are the powerhouses of our cells, essential for the production and management of energy at cell level. Dr Sarah Myhill, together with Dr John McLaren Howard of Acumen Laboratories and Dr Norman Booth of Mansfield College Oxford, has spent many years studying the relationship between their malfunction and the commonest problem seen by GPs in the UK fatigue. Their research findings have been published in three scientific papers in the International Journal of Clinical and Experimental Medicine, in 2009, 2012 and 2013. These studies showed that poor functioning of the mitochondria is the central problem in CFS. Patients with the worst mitochondrial function had the worst fatigue and vice versa. This is solid scientific evidence that CFS is a problem with mitochondria and has allowed the objective measurement of fatigue for the first time. With the publication of the third study, which showed that mitochondrial function tests and symptoms improved in patients who took measures to address their mitochondrial problems, Dr Myhill was ready to write this book. Here she explains the importance of healthy mitochondria, how we can measure their functioning and what we can do to keep them healthy, or restore them to health if problems arise. CFS is all in our cells, not in our minds!