BackgroundImproving health and social care services involves engaging stakeholders in the implementation process. The literature currently reports suboptimal stakeholder engagement in implementation science. Here we draw on the international large-scale ImpleMentAll (IMA) study to illustrate the development of the Implementation-STakeholder Engagement Model (I-STEM) for improving health and social care services. The I-STEM is a sensitising tool, which defines key considerations and activities for undertaking stakeholder engagement activities across an implementation process.MethodsThe IMA project used a stepped-wedged randomised controlled trial design to evaluate the effectiveness of tailored implementation in integrating and embedding evidence-based eHealth interventions in routine care in Europe and Australia. Tailored implementation was operationalised in the ItFits-toolkit, a self-guided platform including resources to support stakeholder engagement (e.g., surveying tool). Here we draw on the qualitative process evaluation that was undertaken alongside the effectiveness trial. We conducted 55 in-depth, semi-structured interviews and observed 19 implementation related activities (e.g., technical support calls) across twelve implementation sites. The analytical process was informed by principles of first and third generation Grounded Theory, including constant comparative method. The I-STEM was derived from the analytical work undertaken in the qualitative process evaluation.ResultsOur findings are presented as the substantive, generalisable I-STEM, consisting of five interrelated concepts: engagement objectives, stakeholder mapping, engagement approaches, engagement qualities, and engagement outcomes. Engagement objectives are goals that implementers plan to achieve by working with stakeholder in the implementation process. Stakeholder mapping involves identifying a range of organisations, groups, or people who may be instrumental in achieving the engagement objectives. Engagement approaches define the type of work that is undertaken with stakeholders to achieve the engagement objectives. Engagement qualities define the logistics of the engagement approach. Lastly, every engagement activity may result in a range of engagement outcomes.DiscussionEffective stakeholder engagement can lead to a better understanding of local needs and barriers and increased research adoption. The I-STEM represents potential avenues for effective stakeholder engagement activity across key phases of an implementation process. It provides a conceptual framework for the planning, delivery, evaluation, and reporting of stakeholder engagement activities. The I-STEM is not prescriptive, but rather highlights the importance of a flexible, iterative approach to stakeholder engagement. It is developmental and will require further application and validation across a range of implementation activities.