Current available anti-inflammatory drugs, in particular monoclonal antibodies directed against the cytokine tumor necrosis factor α (TNF), have greatly enhanced the treatment of inflammatory bowel diseases (IBD). Although many patients respond to ant-TNF therapy, a proportion of patients will not respond (primary non-response) or will lose response to the drug over time (secondary non-response). This loss of response can be caused by patient, TNF-inhibitor, or disease-related factors influencing the pharmacokinetics and pharmacodynamics of the drug. Therefore, monitoring pharmacological parameters (i.e. therapeutic drug monitoring) may help guide therapeutic decisions. This review emphasizes interesting and important new findings, and provides an updated overview, on the subject of therapeutic drug monitoring. While exploring the hypothesis that "one size does not fit all", we focused on the first prospective studies investigating the novel approach in IBD of 'target concentration adjusted dosing' and personalized medicine.