In previous chapters we have considered the development of mentalization up to about the age of 5 years, when the autobiographical self first emerges (see chapter 5). While certain developmental disorders are already apparent at this age, many do not emerge until adolescence or later. This chapter adds a further stage to our description of the development of reflective function, considering the implications of the cognitive developments associated with adolescence for the ontogeny of men talization. The key question addressed in this chapter concerns the increased frequency of various kinds of mental disorder at this developmental stage, the worsening of preexisting conditions, and the emergence of new conditions specifically linked to adolescence, as well as the onset of many that are lifelong problems but whose onset is linked with this age. That is, we attempt to answer the deceptively simple question of why breakdown occurs relatively often in adolescence. We suggest that this might be understood partly in terms of the vicissitudes of reflective function during this developmental phase. The chapter begins with a summary of the view of self-development advocated in this book, which pulls together the threads of the argument so far.