Aryl hydrocarbon receptor (AhR) is an important transcriptional regulator of drug metabolizing enzymes that dominantly controls the expression of cytochrome P450 CYP1 family genes and some phase II enzymes. AhR also has many endogenous functions including cell cycle control, immune response, and cell differentiation. In addition, AhR is well-known to be involved in chemicallyinduced carcinogenesis. AhR is activated by a variety of endogenous and exogenous ligands. While exogenous activation of AhR has deleterious effects on human organism, sustained activation of AhR by endogenous ligands is indispensable for proper cell functions. Therefore, the effects of exogenous and endogenous ligands on AhR resemble the Dr. Jekyll and Mr. Hyde story. The aim of the current paper is to summarize and update the knowledge on exogenous and endogenous AhR ligands.