The ‘improvement’ of health care is now established and growing as a field of research and practice. This article, based on qualitative data from interviews with 21 senior leaders in this field, analyses the growth of improvement expertise as not simply an expansion but also a multiplication of ‘ways of knowing’. It illustrates how health-care improvement is an area where contests about relevant kinds of knowledge, approaches and purposes proliferate and intersect. One dimension of this story relates to the increasing relevance of sociological expertise—both as a disciplinary contributor to this arena of research and practice and as a spur to reflexive critique. The analysis highlights the threat of persistent hierarchies within improvement expertise reproducing and amplifying restricted conceptions of both improvement and ‘better’ health care.