Monoclonal antibodies constitute a therapeutically and economically important drug class, evident by the growing number on the market and in drug development. Still, sub-optimal use of monoclonal antibodies can result in loss of response or adverse events. In this thesis, pharmacometric modelling and simulation methodologies were applied to contribute to the pharmacokinetic/pharmacodynamic understanding of monoclonal antibodies in two populations, namely children and patients with Crohn's disease.