The goal of drug therapy is a defined clinical response. Drug therapy in the vulnerable preterm and term newborn population is complicated by developmental limitations that affect the pharmacokinetics of most drugs. It is important that, in any therapy, care providers maintain a keen awareness of the expected response while monitoring the infant's clinical response throughout drug administration. This column has addressed pharmacologic principles, especially the pharmacokinetics of drug therapy, to guide clinical assessments of patients' responses to drug therapy.