Capturing ontogeny of enzymes involved in phase I metabolism is crucial to improve prediction of dose‐concentration and concentration‐effect relationships throughout infancy and childhood. Once captured, these patterns can be integrated in semiphysiologically or physiology‐based pharmacokinetic models to support predictions in specific pediatric settings or to support pediatric drug development. Although these translational efforts are crucial, isoenzyme‐specific ontogeny‐based models should also incorporate data on variability of maturational and nonmaturational covariates (eg, disease, treatment modalities, pharmacogenetics). Therefore, this review provides a summary of the ontogeny of phase I drug‐metabolizing enzymes, indicating current knowledge gaps and recent progresses. Furthermore, we tried to illustrate that straightforward translation of isoenzyme‐specific ontogeny to predictions does not allow full exploration of scenarios of potential variability related to maturational (non–age‐related variability, other isoenzymes or transporters) or nonmaturational (disease, pharmacogenetics) covariates, and necessitates integration in a “systems” concept.