Crohn's disease (CD) is a chronic inflammatory bowel disease (IBD) with uncertain etiopathogenesis. CD can involve any site of gastrointestinal tract from mouth to anus and is associated with serious complications such as bowel stricture, perforation, abscess, and fistula formation. The incidence and prevalence rates of CD in Korea are still low compared with those of Western countries, but are rapidly increasing during the past decades. The diagnosis of CD is a challenging issue in some clinical situations and it is often difficult to differentiate CD from intestinal tuberculosis or Behçet's enterocolitis which are more prevalent in Korea than in Western countries. Therefore, IBD Study Group of KASID has set out to establish the diagnostic guideline of CD in Korea. There is no single gold standard for the diagnosis of CD and diagnosis is made by clinical evaluation including detailed history, physical examination, and combination of endoscopic findings, histology, radiologic findings and laboratory investigations. The typical symptoms of CD are abdominal pain, diarrhea, and weight loss especially in late adolescence or early adulthood. Initial laboratory investigations include CBC, C-reactive protein, and serum chemistry. Ileocolonoscopy and biopsies are recommended as the first line procedures to establish the diagnosis. Typical endoscopic findings of CD are non-continuous distribution of longitudinal ulcers, cobblestone mucosal appearance, and aphthous ulcerations arranged in a longitudinal fashion. The evaluation of small bowel with small bowel follow-through is recommended for suspected CD to establish diagnosis and to determine the extent and location of disease. Focal and patchy chronic inflammation, focal crypt irregularity, and non-caseating granulomas are usual microscopic features of CD. This is the first Korean diagnostic guideline for CD and needs revision with further data on CD in Koreans.