The Plynlimon catchments are the UK’s most intensively studied long‐term research basins, with 40 years of hydrological and hydrochemical observations. Used to study the impacts of land cover, their establishment, progress and achievements is a complex story of scientific advances in response to national needs, set against a background of academic controversy and competing stakeholder pressures, that remains relevant today. Established to answer the important practical question ‘Do forests use more water than grassland in our climate?’, these research basins helped to revolutionise our fundamental understanding of forest hydrology in Britain, and became a multi‐disciplinary ‘outdoor laboratory’ encompassing floods and droughts, acidification and water chemistry, process studies and climate change used by many universities and research organisations. The findings of the studies there have been widely published, with over 500 papers in refereed journals, and the measurements shared with the international research community. Nevertheless, many data users remain unaware of how and why the catchments were established, the challenges that were overcome, and the ways in which the research programme evolved to meet changing needs. As the study enters its fifth decade, this paper takes the opportunity to look back at its establishment, assess the outputs and offer guidance to other researchers wishing to embark on new catchment or observatory studies.