Major progress has been made in the diagnosis, drug development, and treatment of Crohn's disease and ulcerative colitis. Nevertheless, years after being diagnosed, disease burden remains high for many patients and the proportion of those achieving a complete response is disappointingly low. Moreover, the increase in treatment options disproportionately benefits patients who are less refractory to treatment, because lack or loss of response to previous drugs remains a negative prognostic factor in all new lines of treatment. Patients exposed to other biologics are harder to treat, but there are no accurate estimates of how many individuals fall into this category, and no appropriate treatment guidance exists. No agreed definition for difficult-to-treat ulcerative colitis or Crohn's disease exists. In the setting of longstanding complicated inflammatory bowel disease (IBD), the assessment of severity is complicated by numerous factors, including the presence of extraintestinal manifestations, surgical sequelae, and comorbidities, which hamper treat-to-target and tight control management. To address these issues and identify this population of difficult-to-treat patients, a common operational definition is needed. Alongside improving care, a common definition can also help define patient populations for research—eg, different management strategies and therapeutic agents for difficult-to-treat patients could be compared in clinical trials.