Background: The implementation of youth care guidelines remains a complex process. Several evidence–based frameworks aid the identification and specification of implementation determinants and strategies. However, how certain determinants are influenced by specific strategies remains unclear. Therefore, we need to clarify which active ingredients of strategies, called behaviour change techniques (BCTs), elicit behaviour change and improve implementation outcomes. With this knowledge, we are able to formulate evidence–based implementation hypotheses. An implementation hypothesis details how determinants and in turn, implementation outcomes might be influenced by specific implementation strategies and their BCTs. We aimed to identify 1) determinants relevant to the implementation of youth care guidelines and 2) feasible and potentially effective implementation hypotheses. Methods: A four–round online modified Delphi study was conducted. In the first round, experts rated the implementation determinants based on their relevance. In the second stage, the experts formulated implementation hypotheses by connecting BCTs and implementation strategies to determinants. Next, they were asked to provide a rationale for their choices. In Round 3, the experts reconsidered and finalised their hypotheses based on an anonymous overview of the hypotheses formulated by all the experts, including the substantiations of these hypotheses. Finally, the experts were asked to rate the implementation hypotheses based on their potential effectiveness and feasibility. Results: A total of 14 experts completed the first, second, and third rounds, and 11 completed the final round. Guideline promotion, mandatory education, presence of an implementation leader, poor management support, knowledge regarding guideline use, and a lack of communication skills were reported as most relevant. In total, 46 hypotheses were formulated, ranging from 6 to 9 different hypotheses per determinant. For each determinant, we provide an overview of the implementation hypotheses most often considered feasible and potentially effective.Conclusion: Determinants related to engagement and to knowledge and skills were found to be relevant to youth care guideline implementation. This study offers a set of hypotheses that could help organisations, policymakers, and professionals guide the implementation process of youth care guidelines to ultimately improve implementation outcomes. The effectiveness of these hypotheses in practice remains to be assessed.