The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that functions as an intracellular mediator in the xenobiotic signaling pathway. Although a number of studies have examined AhR-mediated CYP1A1 induction in detail, recent studies of AhR-null mice have revealed that AhR plays important regulatory roles in the normal homeostasis and development of animals. In this short review, we summarize the present state of knowledge about the molecular mechanisms of AhR-mediated CYP1 induction, and we also focus on recent advances in the study of the physiological functions of AhR.