Inflammatory Bowel Diseases (IBD), which include Crohn's disease (CD), ulcerative colitis (UC), and the unclassified form referred to as indeterminate colitis, are most commonly diagnosed during adolescence and early adulthood. In recent decades, an increasing incidence of IBD has been observed in this age group. A wide range of immunomodulatory agents, from corticosteroids to monoclonal antibodies, are now available for the treatment of IBD. These antibodies, known as biologics, target, for example, tumor necrosis factor-alpha (TNF-α; e.g., infliximab and adalimumab), integrin α4β7 (vedolizumab), or interleukin-12/23 (ustekinumab). While infliximab and adalimumab are approved for pediatric use in CD and UC, vedolizumab is only approved for moderate-to-severe UC from the age of 16, and ustekinumab is not approved for pediatric use at all. Nevertheless, vedolizumab and ustekinumab are frequently used off-label in cases of treatment failure with approved therapies, as efficacy has been demonstrated in adult IBD patients, and since 2015, increasing pediatric literature has emerged on their use. To facilitate appropriate dose adjustment in pediatric clinical practice, biologic therapies can be monitored through measurement of drug trough levels. Current pediatric guidelines already recommend incorporating therapeutic drug monitoring (TDM) of infliximab and adalimumab in the management of CD and UC. Studies on TDM for vedolizumab and ustekinumab have so far been conducted almost exclusively in adult IBD patients, where improved treatment responses have also been demonstrated. The presented research is a prospective, non-interventional observational study involving pediatric IBD patients at multiple Austrian pediatric gastroenterology centers. The study duration is five years. The aim is to include at least 40 patients receiving induction and maintenance therapy with infliximab or adalimumab, and 20 patients treated with vedolizumab or ustekinumab during both treatment phases. The primary objective is to gain a better understanding of the pharmacokinetic dynamics of these biologics and the associated treatment response in pediatric settings. Data will be collected exclusively from routine clinical assessments. No additional study-related visits or interventions are planned.