strength-based assessment and intervention directs professionals to identify and build on the strengths and competencies of youth and their families. although this strength-based approach has been used and evaluated within the educational system, there is little research to date on the impact of strength-based planning within the youth criminal justice system. an agency providing detention, custody, and community support services to young offenders in ontario has recently adopted a strength-based approach to programming for youth within their organization. this implementation offered a unique opportunity to study the impact of strength-based programming on juvenile offenders. the current study examined whether youths’ strengths and difficulties change over the course of their incarceration and the variables hypothesized to be associated with this change. these variables include self-efficacy for prosocial behaviours, aggression, and the inhibition of aggression; motivation for change; and attachment to a significant staff worker. parental attachment status was also examined to assess its relationship to strength-based interventions. overall, higher levels of strength were associated with lower levels of difficulty. predictors generally associated with higher levels of strength included lower self-efficacy for aggression, higher self-efficacy for the inhibition of aggression, fewer feelings of alienation from ones worker and decreased motivation for change. higher levels of difficulty were significantly and particularly correlated with higher self-efficacy for aggression, lower self-efficacy for the inhibition of aggression and for prosocial behaviour, and greater motivation for change. maternal attachment was also related to strength and difficulty levels. youth in the current sample did not significantly change in their levels of strength and levels of difficulty over a two-month period, however, open custody youth decreased in their level of difficulty whereas secure custody youth increased. youth who experienced an increase in their level of strength reported fewer feelings of alienation from their significant worker, had lower self-efficacy for aggression, and higher self-efficacy for the inhibition of aggression. self-efficacy for prosocial behaviours increased from time 1 to time 2, whereas attachment to a significant worker variables decreased. limitations and directions for future research are discussed.