The Montreal classification of inflammatory bowel disease (IBD) has been developed to refine the clinical, molecular, and serological classification of IBD. This classification was proposed by a Working Party of experts in 2003, following discussions at the 2005 Montreal World Congress of Gastroenterology. The classification aims to improve the understanding and management of IBD by integrating clinical, molecular, and serological data. For Crohn's disease, the Montreal classification retains the three main parameters: age at diagnosis, disease location, and disease behaviour. However, it introduces new categories for age of diagnosis, such as A1 for early onset (diagnosed at 16 years or younger). The classification also allows for non-mutually exclusive disease locations, reflecting the increasing recognition of upper gastrointestinal involvement in Crohn's disease. Additionally, the classification distinguishes between perianal and abdominal fistulising disease, as perianal disease may not necessarily correlate with intestinal fistulising disease. For ulcerative colitis, the Montreal classification defines disease extent into three subgroups and classifies disease activity into four categories. The classification emphasizes the dynamic nature of disease, with changes in extent and behaviour over time. The classification also considers the risk of colorectal malignancy, which supports its use in clinical practice. Indeterminate colitis is defined as a condition where the diagnosis cannot be made with certainty based on clinical and endoscopic findings. The Montreal classification recommends using the term "inflammatory bowel disease, type unclassified" (IBDU) for such cases, rather than "indeterminate colitis." The classification also incorporates serological and genetic markers to refine the diagnosis and classification of IBD. While serological markers such as ASCA and p-ANCA are useful, their clinical utility is limited due to variability in seroprevalence and assay sensitivity. Genetic markers, such as NOD2/CARD15, are associated with specific disease phenotypes, but their use in classification is still under investigation. The Montreal classification aims to improve the accuracy of IBD diagnosis and management by integrating clinical, molecular, and serological data. However, further research is needed to validate the classification and assess its utility in clinical practice. The classification also highlights the importance of considering genetic and environmental factors in the management of IBD.The Montreal classification of inflammatory bowel disease (IBD) has been developed to refine the clinical, molecular, and serological classification of IBD. This classification was proposed by a Working Party of experts in 2003, following discussions at the 2005 Montreal World Congress of Gastroenterology. The classification aims to improve the understanding and management of IBD by integrating clinical, molecular, and serological data. For Crohn's disease, the Montreal classification retains the three main parameters: age at diagnosis, disease location, and disease behaviour. However, it introduces new categories for age of diagnosis, such as A1 for early onset (diagnosed at 16 years or younger). The classification also allows for non-mutually exclusive disease locations, reflecting the increasing recognition of upper gastrointestinal involvement in Crohn's disease. Additionally, the classification distinguishes between perianal and abdominal fistulising disease, as perianal disease may not necessarily correlate with intestinal fistulising disease. For ulcerative colitis, the Montreal classification defines disease extent into three subgroups and classifies disease activity into four categories. The classification emphasizes the dynamic nature of disease, with changes in extent and behaviour over time. The classification also considers the risk of colorectal malignancy, which supports its use in clinical practice. Indeterminate colitis is defined as a condition where the diagnosis cannot be made with certainty based on clinical and endoscopic findings. The Montreal classification recommends using the term "inflammatory bowel disease, type unclassified" (IBDU) for such cases, rather than "indeterminate colitis." The classification also incorporates serological and genetic markers to refine the diagnosis and classification of IBD. While serological markers such as ASCA and p-ANCA are useful, their clinical utility is limited due to variability in seroprevalence and assay sensitivity. Genetic markers, such as NOD2/CARD15, are associated with specific disease phenotypes, but their use in classification is still under investigation. The Montreal classification aims to improve the accuracy of IBD diagnosis and management by integrating clinical, molecular, and serological data. However, further research is needed to validate the classification and assess its utility in clinical practice. The classification also highlights the importance of considering genetic and environmental factors in the management of IBD.