||Key learning points Appreciate the mechanisms by which anions can trigger colour changes in small molecule receptors containing hydrogen bond donor groups. Understand how excimer formation can be used to sense anionic guests by bringing about conformational change or aggregation in fluorescent receptors. Understand the different roles metals and Lewis acids can play in selective anion sensing. Appreciate how the displacement assay approach allows receptors that do not contain signalling groups to act as sensors for anions. Understand that receptors can be used in arrays together with pattern recognition techniques to distinguish mixtures of anions.