BACKGROUND: Previous studies have detailed the technical, learning and soft skills healthcare staff deploy to deliver quality improvement (QI). However, research has mainly focused on management and leadership skills, overlooking the skills frontline staff use to improve care. Our research explored which skills mattered to frontline health practitioners delivering QI projects. STUDY DESIGN: We used a theory-driven approach, informed by communities of practice, knowledge-in-practice-in-context and positive deviance theory. We used case studies to examine skill use in three pseudonymised English hospital Trusts, selected on the basis of Care Quality Commission rating. Seventy-three senior staff orientation interviews led to the selection of two QI projects at each site. Snowball sampling obtained a maximally varied range of 87 staff with whom we held 122 semistructured interviews at different stages of QI delivery, analysed thematically. RESULTS: Six overarching 'Socio-Organisational Functional and Facilitative Tasks' (SOFFTs) were deployed by frontline staff. Several of these had to be enacted to address challenges faced. The SOFFTs included: (1) adopting and promulgating the appropriate organisational environment; (2) managing the QI rollercoaster; (3) getting the problem right; (4) getting the right message to the right people; (5) enabling learning to occur; and (6) contextualising experience. Each task had its own inherent skills. CONCLUSION: Our case studies provide a nuanced understanding of the skills used by healthcare staff. While technical skills are important, the ability to judge when and how to use wider skills was paramount. The provision of QI training and fidelity to the improvement programme may be less of a priority than the deployment of SOFFT skills used to overcome barriers. QI projects will fail if such skills and resources are not accessed.