Faced with the triple planetary crises of climate change, pollution and biodiversity loss, states are increasingly taking regulatory action, especially in the natural resources and energy sectors, to address these pressing environmental problems. This regulatory action can impact financially on investments in the host state. In response, investors are resorting to investor–state dispute settlement (ISDS) clauses in investment treaties between states, claiming compensation for the financial impact of the regulatory action. This increase of ISDS claims is controversial. Amongst other criticisms, the bringing of ISDS claims, or even the threat of ISDS claims, has a chilling effect on states taking needed regulatory action to address pressing environmental problems. There have been calls for reform of the ISDS system, including its application to environmental regulation. This article maps the nature, history and evolution of ISDS claims in natural resources, energy and other environmental cases and the controversies these ISDS claims have attracted. The article canvasses the calls for reform of the ISDS system.