This paper focuses on our experience of researching the influence of ResourceSmart Schools, a sustainable schools programme in Victoria, Australia. Drawing on ideas from programme theory and realist synthesis, we illustrate and reflect upon our approach to conceptualising, investigating and generating evidence about the programme's "impacts" and "influence" in participating schools. This distinction is deliberate: it helps distinguish between efforts to understand the impacts that a programme has within schools ("programme impact"), and efforts to understand what it is about a programme that is influential in bringing about those impacts ("programme influence"). Drawing on evidence from our work in this project and the wider literature, we argue for a more nuanced discussion and more sophisticated investigations into the complexities of programme influence, rather than impacts only. Our conclusions suggest key areas of development for our own work, the provision of environmental and sustainability education, and their evaluation and research more broadly.