Crohn's disease (CD) and ulcerative colitis (UC) are the two main types of chronic inflammatory bowel disease (IBD). Despite recent advances, many patients do not respond to available treatments and or lose response over time. In 2023, the International Organisation for the Study of IBD (IOIBD) proposed a common definition of 'difficult-to-treat' inflammatory bowel disease (IBD-IBD) to homogenise terminology and promote research into patients most in need of new treatments and therapeutic strategies. According to the IOIBD criteria, IBD is defined by any of the following: failure of two or more advanced treatments with different mechanisms of action, postoperative recurrence of Crohn's disease after two or more bowel resections, pouchitis refractory to antibiotics, complex perianal Crohn's disease, or the presence of psychiatric comorbidity that prevents adequate therapeutic management. As the definition of DTT-IBD is very recent, the prevalence and risk factors of DTT-IBD are not yet known. This study aims to determine the prevalence of DTT-IBD in the patient population and the risk factors associated with the development of DTT-IBD. The study will be conducted as a retrospective cross-sectional study in two large tertiary care centers, IRCCS Ospedale San Raffaele and IRCCS Humanitas Research Hospital, both in Milan, Italy. The study will evaluate the criteria and risk factors for DTT-IBD in the latest available gastroenterological examination report, provided it was performed in the last 5 years (from 1 January 2019).