The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that has a crucial role in maintaining intestinal homeostasis. In particular, AHR ligands such as dietary components and microbiota metabolites are required for the development, maintenance and functions of intestinal intraepithelial lymphocytes (IELs). However, Panda et al. now show that despite these essential functions, AHR activity must be carefully regulated to prevent oxidative stress and death of IELs.