Growth and development are two major aspects of children not readily apparent in adults. Clearance in the paediatric population should be investigated using models that describe size, maturation and organ function influences. Size is the primary covariate and although lean body weight is argued as a better measure than total body weight, the use of different fractions of fat mass to explain how pharmacokinetic parameters vary with body composition has been proposed. Allometric scaling using an empiric power exponent of 3/4 is superior to scaling using body surface area. The sigmoid hyperbolic model has proven useful to describe maturation. An extra parameter that describes asymmetry can be incorporated into this model. These descriptors are used to illustrate creatinine, morphine and paracetamol clearance in children. Simultaneous investigation of pooled GFR, paracetamol and morphine data enabled testing for shared common features of maturation processes. Results suggest that GFR matures before paracetamol or morphine clearance, consistent with phase II conjugation processes that convert xenobiotics to water soluble forms that can subsequently be eliminated from the body through the renal system. The use of such mechanistic approaches improves understanding of paediatric pharmacokinetics; improving dosing predictions and allowing projection in exploratory drug development.