Clinical trials are more difficult to conduct in children, but they are even more necessary than in adults their scarcity is an ethical scandal. Mathematical models can be built that can describe both the disease process and the mechanism of action of drugs. These models can then be used to simulate the outcome of clinical trials. Inspection of the simulated results then facilitates optimisation of the trial design and proposed methods of analysis. Validation is a crucial issue for the good practice of modelling and simulation. The participants of Round Table No. 6 recommend: (i) that modelling be systematically employed; (ii) that all the required professional personnel be involved, at all phases; (iii) that all data needed are made accessible; (iv) that clinicians be trained; (v) that specialists develop training tool kits; and (vi) that universities provide appropriate training.