Abstract Background: Multiple and complex needs (MCN) in children and youth jeopardize their development and pose significant challenges to the different professionals they meet. However, there is no agreed-upon definition of this vulnerable population. Objectives: To develop a definition of ‘MCN in children and youth’ that is meaningful for all professionals involved in care delivery for this population. Method: A cross-sector, multidisciplinary, and geographically spread panel of 47 experts representing mental health, youth care, juvenile justice, and education in Flanders participated in an online Delphi study. Qualitative analysis of answers in the first round yielded four definition possibilities that participants then ranked in the second round. In the last round, participants rated their agreement with the highest ranked definition. An additional survey asked 25 international experts to rate and comment their agreement with the final definition. Results: The final definition was: Children and adolescents with profound and interacting needs in the context of issues on several life domains (family context, functioning and integration in society) as well as psychiatric problems. The extent of their needs exceeds the capacity (expertise and resources) of existing services and sequential interventions lead to discontinuous care delivery. As such, existing services do not adequately meet the needs of these youths and their families. Cross-sector, integrated and assertive care delivery is necessary for safeguarding the wellbeing, development and societal integration of these young people. Response rates to the three Delphi rounds were 76.6, 89.1, and 91.3%. The definition was widely endorsed among Flemish (93.2% agreement) and international experts (88% agreement). Conclusion: A definition of MCN in children and youth was constructed using the Delphi method and further evaluated for international relevance in an additional survey. Such an agreed-upon definition can be valuable for optimizing care delivery and ...