Reducing inequalities in health and the determinants of health is a widely acknowledged health policy goal, and methods for measuring inequalities and inequities in health are well developed. Yet, the evidence base is weak for how to achieve these goals. There is a lack of high-quality randomised controlled trials reporting impact on the distribution of health and non-health benefits and lack of methodological rigour in how to design, power, measure, analyse and interpret distributional impact in RCTs. Our overarching aim in this paper is to contribute to the emerging effort to improve transparency and coherence in the theoretical and conceptual basis for RCTs on effective interventions to reduce health inequity. We endeavour to achieve this aim by pursuing two more specific objectives. First, we propose an overview of three broader health equity frameworks and clarify their implications for the measurement of health inequality in RCTs. Second, we seek to clarify the relationship between theory and translational challenges that researchers would need to attend to, in order to ensure that equity-relevant RCTs are coherently grounded in theory.