Capacity and competence in the field of child and adolescent psychiatry are complex issues, because of the many different influences that are involved in how children and adolescents make treatment decisions within the setting of mental health. This article will examine some of the influences which must be considered, namely: developmental aspects, the paradoxical relationship between the need for autonomy and participation and the capacity of children, family psychiatry, and the duty of care towards children and adolescents. The legal frameworks relevant to consideration of consent and competence will be briefly considered, as well as some studies of children's consent, participation and competence. A case vignette will be used as a focus to consider the complexity of the issue of competence in child and adolescent psychiatry, in the particular mental disorder of anorexia nervosa.