Since the 1990s, there have been repeated calls for the systematic use of randomised controlled trials (RCTs) to inform educational decision-making. The advent of the Education Endowment Foundation (EEF) -- described as England's What Works Centre for Education -- in 2011 has made this a reality in England: by 2020, over a third of English schools were involved in such trials. Despite much debate about the value and role of RCTs, less attention has been paid to one specific effect of such trials, a phenomenon we refer to as interventionisation. This article uses two examples, focused on language and literacy education and teacher professional development, to demonstrate how increased use of trials may work to 'interventionise' education through channelling the focus of innovation and development to tightly structured interventions and generating a series of narrowing effects. It argues that a broad view of research and a diversity of methodologies is needed not only to generate rich understandings of educational practice but to develop and sustain educational provision that is fit for a dynamic world and which responds to the challenges and opportunities presented in complex educational contexts.