The idea of a critical load (CL), i.e., a deposition limit that would protect sensitive ecosystems in the long run, was first suggested by Canadian scientists during the early 1980s. The concept was taken over and further developed in Scandinavia during the mid-80s and by the UNECE Convention on Long-range Transboundary Air Pollution (CLRTAP) towards the end of the 1980s. Critical loads were subsequently adopted by the CLRTAP as a tool to inform and guide negotiations on emission reduction protocols. At this time a Task Force on Modelling & Mapping, together with a Coordination Centre for Effects (CCE), was also set up under the CLRTAP, with the mandate to compile, maintain and update CL models and databases. Models and methods were documented in a so-called Mapping Manual, which was/is regularly updated and functions as a guideline for a country’s National Focal Centre (NFC) to determine internationally compatible CL data for use under the CLRTAP. The negotiations of the Second Sulphur Protocol (1994) marked the first use of CLs, and only five years later the Gothenburg Protocol was signed, following negotiations supported by both CLs for acidification and eutrophication. Updated CLs were used in the revision of the Gothenburg Protocol (2012); further, the European Union (EU) adopted CLs in their negotiations of the latest National Emission Ceilings (NEC) Directive. More recently, the Convention has supported and promoted the development of CLs of sulphur (S) and nitrogen (N) that have biodiversity as an endpoint, given the increasing importance of biological diversity (and the potential impacts from air pollution) . This presentation will give an overview of these various stages in the CL development, and also compare it to developments outside Europe, taking Canada as an example.