This thesis aims to understand the practical arrangements which families resort to when they face the mental difficulties of a child or a teenager. This research, based on the ethnographic study of 42 families living in the Paris region and on local and national statistical data, emphasises the diagnostic theories which the child's relatives put forward in order to explain, describe and anticipate his or her behaviour. These theories are considerably influenced by professional discourses but they are at the heart of various stakes: power struggles with health professionals, with people working with disabled children, and with the child's family circle. The decisions made by these teenagers鈥?relatives and the way they vary from one situation to the next can be better understood by disentangling the relationship between those stakes and diagnostic theories.