While patient and public involvement (PPI) in health and social care research has progressed successfully in the last decade, a range of difficulties with the evidence base exist, including poor understanding of the concept of impact, limited theorization and an absence of quantitative impact measurement. In this paper, we argue that a pardigm change towards robust measurement of the impact of involvement in research is needed to complement qualitative explorations. We argue that service users should be collaboratively involved in the conceptualization, theorization and development of instruments to measure PPI impact. We consider the key advantages measurement would bring in strengthening the PPI evidence base through a greater understanding of what works, for whom, in what circumstances and why.