While much is known about reactions and substrates of xenobiotic metabolizing enzymes, their specificity towards xenobiotics, drugs and endogenous substrates like steroid hormones is less well understood. Protein-protein interactions also influence the substrate recognition and metabolism. The topics will address whether drug and steroid metabolizing enzymes can recognize different classes of substrates, how the substrate selection and activities are influenced and whether ambiguity in substrate recognition leads to unexpected metabolic activities.