Recent advances in pediatric clinical pharmacology have provided a more rational approach to using several medications in children. An increased understanding of the effect of human development, concurrent medications, organ function, and disease states on the absorption, distribution, metabolism, and excretion of drugs has provided a stronger scientific basis for determining drug dosages in children. By measuring drug concentrations and utilizing pharmacokinetic and pharmacodynamic principles, the probability of therapeutic response can be enhanced for a number of medications. Likewise, therapeutic drug monitoring can minimize the risk of adverse effects from many drugs used in children. However, it must be recognized that toxicity can occur in some patients even though plasma drug concentrations are in the therapeutic range; similarly, some patients may not experience a therapeutic effect when plasma drug concentrations are in the same target range. Therefore, achieving the desired plasma concentration of a drug can enhance both the probability of a therapeutic response and diminish the probability of a toxic response. Therapeutic ranges, however, are only intermediate endpoints that must be used in the context of additional criteria to assess the clinical efficacy of any given drug therapy.