The significance of a process evaluation for understanding randomised controlled trials (RCTs) of complex teaching interventions in a classroom setting is discussed in relation to the evaluation of the Integrated Group Reading (IGR) programme trial. This process evaluation is based on realist principles and draws on different data sources. The findings indicate that the efficacy of a classroom-based complex intervention like IGR is hard to evaluate given the nature of the intervention and its context. These findings illustrate that a process evaluation should not be seen as a subsidiary study to the experimental trial, but as an independent study in its own right as it can shed light onto the setting up and running of the trial. The paper argues that RCTs by themselves do not necessarily provide the 'best evidence' and need to be supplemented by other evaluation perspectives.