Therapeutic drug monitoring (TDM) has emerged as a strategy for treatment optimization in inflammatory bowel diseases in order to maximise benefit and to reach more stringent, objective endpoints. Optimal drug concentrations in IBD vary according to treatment target, disease phenotype, inflammatory burden and timing of sampling during the treatment cycle. This review provides an update on TDM with biologic and oral small-molecules, evaluates the role of reactive versus proactive TDM and identifies the gaps in current evidence.