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Evidence-based interventions (EBIs) are effective tools for improving and maintaining health and wellbeing across various settings in society. However, their success relies on balancing adherence to the components and core elements of EBIs that ensure effectiveness with the need to adapt these interventions to fit different contexts. This challenge, known as the fidelity-adaptation dilemma, is highly relevant in practice during the sustainment phase of implementation, where professionals delivering EBIs to target groups are responsible for managing this dilemma. Even though EBIs are a central part of many professionals' working-life, there is limited understanding of how professionals experience the dilemma in practice and how it is associated with their psychosocial working conditions. There is also a gap in knowledge about the sustainment phase, during which professionals are key to the success of EBIs—in other words, they determine to a high degree whether an EBI is effective for its target group through the adaptations they make. Moreover, little is known about how decision support interventions can help professionals balance fidelity and adaptation. The overall aim of this thesis is therefore to explore the fidelity-adaptation dilemma from the professionals' perspective by looking at their perceptions, attitudes, management, and psychosocial working conditions when using EBIs in the sustainment phase of implementation. A secondary aim is to investigate a structured decision support intervention—the Adaptation and Fidelity Tool (A-FiT)— that aims to support professionals in making planned, intentional decisions about fidelity and adaptations.

This thesis comprises four studies, each focusing on different main aspects of professionals' experiences with the fidelity-adaptation dilemma when using EBIs in practice: perceptions (Study I), attitudes (Study II), and psychosocial working conditions (Studies II and IV) but also adaptations made (Study III), and the investigation of a decision support intervention (Studies III and IV). The participants were professionals working with two evidence-based parent education programs in Sweden: Cool Kids (Study I) and All Children in Focus (ABC) (Studies II–IV). The research employed both qualitative and quantitative methods, with cross-sectional and longitudinal designs.

The key findings of the thesis indicate that professionals have varying attitudes toward fidelity and adaptation (Studies I and II) but generally perceive the fidelity-adaptation dilemma as challenging (Study I). Professionals who favored adaptations over fidelity reported more demanding working conditions compared to those who preferred fidelity or valued both equally (Study II). Prompts encouraging reflection on managing fidelity and adaptations were associated with lower perceived skills in delivering the EBI ABC (Study IV). The results also reveal that all professionals made adaptations (Studies I and IV), primarily to better fit the EBI into the context (practical adaptations). While the professionals made about the same number of adaptations both before and after participating in A-FiT, the reasons for making them changed. Practical adaptations increased in number after the professionals participated in A-FiT, while adaptations aimed at improving the EBI's effectiveness decreased (Study III).

This thesis advances the understanding of the fidelity-adaptation dilemma, demonstrating how EBIs can act as both resources and job demands. The findings highlight the importance of reflection both before, during (in), and after (on) action in making effective decisions about fidelity and adaptation, as supported by A-FiT and reflective prompts. Addressing this dilemma requires support beyond the individual level, including education and information about EBIs' components, fidelity, adaptations, and implementation, as well as organizational support and adequate resources, such as opportunities and time to reflect. The thesis emphasizes the need for a balanced approach to fidelity and adaptation for effective EBI sustainment, and calls for further investigation of the dilemma from the professionals' perspective and the impact of tools such as AFiT. Such research, together with the findings from this thesis could contribute to better support for professionals in their management and experience of the dilemma and to that society gains the best possible outcomes from EBIs in the future.