Drug evaluation in children is difficult for many well-identified reasons and many drugs are still used off-label. Innovative approaches are particularly adapted to the paediatric and neonatal populations, as clinical trials are difficult to conduct, need adapted designs in order to define the optimal dosage regimen in many diseases and therapeutic areas. Population approaches to define pharmacokinetics and pharmacokinetic/pharmacodynamics are now more currently used to define dosing regimens, adapted to the different paediatric and neonatal age groups, that allow to increase efficacy and reduce toxicity, by taking into account factors explaining variability in drug response. Such approaches are presented and the evaluation of vancomycin in neonates is detailed as different steps allowed validation of the optimal strategy to administer vancomycin in neonates.