Concerns with the prevalence of sedentariness and obesity in children and adolescents have provided the impetus for a worldwide explosion in articles advocating the routine surveillance of cardiorespiratory fitness (CRF) in these age groups based on purported associations between improved CRF and the prevention of lifetime metabolic and cardiovascular disease. This concern with CRF in healthy youth is paralleled by a proliferation of research into the CRF of young people with severe, life-limiting, or disabling conditions. A fairly cursory search of the medical literature suffices to reveal recent reports on CRF measurements in children and adolescents with anorexia nervosa, cystic fibrosis, cerebral palsy, muscular dystrophies, congenital heart disease, renal transplant, severe burns, ciliary dyskinesia, genetic disorders, and other conditions. Not surprisingly, given population trends, there are also numerous studies of CRF measurements in children with obesity.