Participation in organized, competitive sport often begins as early as 6-7 years of age and by their teens some young people have experienced several years of intensive training and high-level competition. Initial selection for, and retention in, elite sport takes place within a matrix of biocultural characteristics, which include health status, family size, parental support, socio-economic status, and psychological readiness. However, performance in sport during childhood and adolescence is underpinned by a range of physical, biochemical and physiological factors which operate in a sport-specific manner and are dependent on individual biological clocks. Experimental techniques initially pioneered with adults and new non-invasive technologies have been successfully modified for use with children and adolescents. These developments have opened up new avenues of research into exercise performance during youth but data on elite young athletes are sparse. This paper will briefly reference what we do and don't know in key areas in order to inform and challenge paediatric exercise scientists involved in supporting elite young athletes.