Background: In 2012, Alberta Health Services created Strategic Clinical NetworksTM (SCNs) to develop and implement evidence-informed, clinician-led and team-delivered health system improvement in Alberta, Canada. SCNs have had several provincial successes in improving health outcomes. Little research has been done on the sustainability of these evidence-based implementation efforts.Methods: We conducted a qualitative realist evaluation using a case study approach to identify and explain the contextual factors and mechanisms perceived to influence the sustainability of two provincial SCN evidence-based interventions, a delirium intervention for Critical Care and an Appropriate Use of Antipsychotics (AUA) intervention for Senior’s Health. The context (C) + mechanism (M) = outcome (O) configurations (CMOcs) heuristic guided our research.Results: We conducted thirty realist interviews in two cases and found four important mechanisms facilitating sustainability: the use of a collaborative approach, audit & feedback, the informal leadership role, and patient stories. Informal leaders were often hands-on and influential to front-line staff. Learning collaboratives broke down professional and organizational silos and encouraged collective sharing and learning, motivating participants to continue with the intervention. Continual audit & feedback interventions motivated participants to want to perform and improve on a long-term basis, increasing the likelihood of sustainability of the two scaled, multi-component interventions. Patient stories demonstrated the interventions’ impact on patient outcomes, motivating staff to want to continue doing the intervention, and increasing the likelihood of its sustainability.Conclusions: There are important contextual factors and mechanisms within sustainability processes that may apply to systems change implementers and decision makers. Our research revealed the causal relationship between implementation and sustainability and how outcomes from implementation shape sustainability contexts. Future work is needed to evaluate the effectiveness of informal leadership, learning collaboratives, audit-feedback, and patient stories as strategies for sustainability, to generate better guidance on planning sustainable improvements with long term impact.