The inclusion of children within epidemiological studies has been sustained by three main protective measures: proxy consent, assent and research classified as of ‘minimal risk’. These have raised issues centring on when the child should be considered able to decide on such participation. Studies on longitudinal birth cohort studies reveal that there exists a tension surrounding such issues. Since autonomy is regarded as necessary for informed consent, researchers have sought developmental theories that map cognitive abilities along a biological trajectory towards autonomy. Consequently, children’s competencies are seen as largely determined by natural causes. This book draws attention to the social constructiveness of concepts central to the child’s decisional power. This is to underscore that technologies, such as biobanks, play a role in defining ‘childhood’, through the provision of a novel context in which to define the competencies of the child.