BackgroundAlthough sustainability remains a recognised challenge for Quality Improvement (QI) initiatives, most available research continues to investigate sustainability at the end of implementation. As a result, the learning and continuous adjustments that shape sustainability outcomes are lost. With little understanding of the actions and processes that influence sustainability within QI initiatives, there is limited practical guidance and direction on how to enhance the sustainability of QI initiatives. This study aims to unpack the ‘process of sustaining’, by exploring threats to sustainability encountered throughout the implementation of QI Initiatives and identifying strategies used by QI teams to address these threats over time.MethodsA longitudinal multiple case study design was employed to follow 4 QI initiatives over a 3-year period. A standardised sustainability tool was used quarterly to collect perceptions of sustainability threats and actions throughout implementation. Interviews (n=38), observations (32.5 h), documentary analysis, and a focus group (n=10) were conducted to enable a greater understanding of how the process of sustaining is supported in practice. Data were analysed using the Consolidated Framework for Sustainability (CFS) to conduct thematic analysis.ResultsAnalysis identified five common threats to sustainability:workforce stability, improvement timelines, organisational priorities, capacity for improvement, and stakeholder support. Each of these threats impacted multiple sustainability constructs demonstrating the complexity of the issues encountered. In response to threats, 12 strategies to support the process of sustaining were identified under three themes:engagement(five strategies that promoted the development of relationships),integration(three strategies that supported initiatives to become embedded within local systems), andadaptation(four strategies that enhanced understanding of, and response to, emergent conditions and contextual needs).ConclusionsSustaining improvements from QI initiatives requires continuous investment in relationships, resilience to integrate improvements in local systems, and flexibility to understand emergent conditions. Findings provide practitioners, funders, and researchers with a better understanding of, and preparation for, the threats associated with sustaining improvements from QI initiatives and offer insight into specific actions that can be taken to mitigate these risks. This learning can be used to inform future initiative design and support, to optimise the sustainability of healthcare improvements.Trial registrationNot applicableSupplementary InformationThe online version contains supplementary material available at 10.1186/s43058-023-00445-z.