Self-understanding is a fundamental aspect of psychological functioning. This study aimed to explore self-understanding in adolescents with autism spectrum conditions (ASC), adolescents with Down syndrome (DS) and adolescents with Williams syndrome (WS). The Self-understanding Interview was used to examine the verbal concepts and opinions of self of 45 adolescents (15 ASC, 15 DS and 15 WS). Adolescents with ASC generated significantly fewer social statements compared with adolescents with WS. There was a close comparability between adolescents with ASC, DS and WS on the number of self-as-object self-statements indicating similarities in the overall development of self-concept. The development of social self-understanding and its implications were discussed. The need for individually tailored interventions enhancing social skills promoting positive self-concept is highlighted.