 For many years, it has been considered dogma that drug biotransformation capability is limited at best in the fetus and newborn but increases over the first year of life to levels in toddlers and young children that generally exceed adult capacity. There are several situations where examination of clinical PK data has revealed discernable patterns of drug clearance that can be attributed to developmental differences in drug biotransformation. It has become apparent that there are developmental differences in expression among drug metabolizing enzyme families cytochromes P450 or CYPs, etc. Furthermore, individual drug metabolizing enzymes with in a family may have unique developmental profiles that influence the therapeutic response, desired or undesired, to a given agent. 