Unexplained variations in organisational performance in healthcare are a continued focus of research, political, and public interest. Regulatory agencies are always seeking new ways to reduce variation and improve performance, and the use of approaches to develop improvement capability is increasingly encouraged. However, the regulatory perspective on improvement capability is under researched.This research study seeks to understand how regulatory agencies in the United Kingdom (UK) assess improvement capability within healthcare organisations. It explores how improvement capability is conceptualised, compares the regulatory arrangements across the UK, and examines assessment and enforcement policies and practices, before developing a conceptual framework for improvement capability.The research study uses data from 48 interviews, 90 regulatory policy documents and 30 assessment reports. Regulatory conceptualisations of improvement capability are explored through cross-case comparison and qualitative analysis. A review of 70 instruments and frameworks for the assessment of improvement capability from the literature identifies that there are plural conceptualisations of improvement capability. The findings from the review are synthesised into eight dimensions of improvement capability which are used to analyse the empirical data and to develop a conceptual framework.The analysis finds an emergent trend towards responsive regulatory models which aim to develop improvement capability. However, the analysis identifies ambiguity in regulatory agencies' conceptualisation of improvement capability with two dimensions of improvement capability used more frequently than others in regulatory assessments. Regulatory agencies need to clarify their conceptualisation of improvement capability and supplement their assessment processes to further understand local circumstances. This can be used to inform more flexible regulatory responses, including the tailored provision of improvement support to develop improvement capability. This requires greater regulatory effort and resources, and the analysis finds there are three areas of tension, linked to regulatory roles, resources and relationships. The research study proposes a conceptual framework of improvement capability that can be used to clarify regulatory conceptualisation and assessment of improvement capability. Greater conceptual clarity will strengthen regulatory agencies' assessment, diagnosis and prediction of organisational performance trajectories, and support the advancement of more appropriate, effective and responsive regulatory interventions, including the development of improvement capability.