SummaryUV‐B, UV‐A and blue light control a variety of aspects of plant development via distinct photoreceptors and signalling pathways. The known photoreceptors for UV‐A/blue light are cryptochrome (cry)1 and cry2, and the phototropism photoreceptor, phototropin. Redox processes are important in cry and phototropin signal transduction. A specific photoreceptor for UV‐B has not been identified and there appear to be several possible UV‐B signalling pathways. We are investigating the UV and blue light regulation of transcription of the chalcone synthase gene (CHS) in Arabidopsis. Experiments with photoreceptor mutants show that distinct UV‐A/blue (cry mediated) and UV‐B photoreception systems control CHS expression. Experiments with an Arabidopsis cell suspension culture show that the UV‐B and cry1 signalling pathways differ kinetically and pharmacologically. In contrast to some other UV‐B responses, the UV‐B induction of CHS does not appear to involve oxidative stress signalling. Promoter elements and candidate transcription factors that effect CHS induction have been identified. Interactions within a network of UV‐B, cry and phytochrome signalling pathways regulate CHS expression. Synergistic interactions between the UV‐B pathway and distinct UV‐A and blue‐light pathways maximize the response. In addition, specific phytochromes positively control the cry1 pathway via distinct potentiation and coaction effects, and negatively regulate the UV‐B pathway.