The article discusses the Montreal classification of inflammatory bowel disease (IBD), focusing on the key issues and controversies identified by a Working Party of investigators. The classification aims to integrate clinical, molecular, and serological aspects to improve patient counseling, prognosis assessment, and therapy selection. The review highlights the following: 1. **Clinical Classification of Crohn's Disease**: The Montreal classification revises the Vienna classification by introducing a new category for early-onset disease (A1) and allowing for non-mutually exclusive upper gastrointestinal involvement. It also emphasizes the dynamic nature of disease behavior, particularly in perianal fistulizing disease. 2. **Ulcerative Colitis**: The classification focuses on disease extent and severity, recognizing the instability of disease extent over time. It proposes a classification system based on the maximal extent of involvement and severity of relapse. 3. **Indeterminate Colitis**: The term "indeterminate colitis" is reserved for cases where colectomy has been performed and a definitive diagnosis cannot be made. A new term, "inflammatory bowel disease, type unclassified" (IBDU), is suggested for patients with chronic inflammatory bowel disease affecting the colon without small bowel involvement. 4. **Molecular Diagnosis**: While an integrated molecular classification is not yet justified, the Working Party discusses the potential of serological and genetic markers to refine disease classification. Serological markers like p-ANCA and ASCA are limited in sensitivity and specificity, but novel markers like anti-OMPC and anti-I2 show promise. Genetic markers, such as NOD2/CARD15, are associated with ileal Crohn's disease and complex disease behavior but are not yet sufficient for routine diagnosis. 5. **Research Agenda**: The Working Party emphasizes the need for independent validation and a minimal dataset for research studies to advance the field of IBD classification and understanding. The article concludes by highlighting the need for parallel prospective studies and the potential for an integrated classification within the next 5-10 years.The article discusses the Montreal classification of inflammatory bowel disease (IBD), focusing on the key issues and controversies identified by a Working Party of investigators. The classification aims to integrate clinical, molecular, and serological aspects to improve patient counseling, prognosis assessment, and therapy selection. The review highlights the following: 1. **Clinical Classification of Crohn's Disease**: The Montreal classification revises the Vienna classification by introducing a new category for early-onset disease (A1) and allowing for non-mutually exclusive upper gastrointestinal involvement. It also emphasizes the dynamic nature of disease behavior, particularly in perianal fistulizing disease. 2. **Ulcerative Colitis**: The classification focuses on disease extent and severity, recognizing the instability of disease extent over time. It proposes a classification system based on the maximal extent of involvement and severity of relapse. 3. **Indeterminate Colitis**: The term "indeterminate colitis" is reserved for cases where colectomy has been performed and a definitive diagnosis cannot be made. A new term, "inflammatory bowel disease, type unclassified" (IBDU), is suggested for patients with chronic inflammatory bowel disease affecting the colon without small bowel involvement. 4. **Molecular Diagnosis**: While an integrated molecular classification is not yet justified, the Working Party discusses the potential of serological and genetic markers to refine disease classification. Serological markers like p-ANCA and ASCA are limited in sensitivity and specificity, but novel markers like anti-OMPC and anti-I2 show promise. Genetic markers, such as NOD2/CARD15, are associated with ileal Crohn's disease and complex disease behavior but are not yet sufficient for routine diagnosis. 5. **Research Agenda**: The Working Party emphasizes the need for independent validation and a minimal dataset for research studies to advance the field of IBD classification and understanding. The article concludes by highlighting the need for parallel prospective studies and the potential for an integrated classification within the next 5-10 years.