Many anti-cancer drugs have a narrow therapeutic window and are associated with significant toxicity. Current dosing methods are generally inadequate and may result in either life-threatening toxicity or impaired disease control. This is particularly evident in babies and young children. Correct dosing is also a challenge for the adolescent and young adult population due to pubertal changes. Through studying the pharmacokinetics (PK, dose vs. concentration relationship) and pharmacodynamics (PD, concentration vs. effect relationship) of these drugs it is possible to identify optimal exposure targets that are associated with reduced toxicity and enhanced efficacy (PK/PD research) and to adjust the dose accordingly. Further research is required to test whether the dose adjustments do actually lead to reduced toxicity and improved efficacy.