Ulcerative colitis and Crohn disease may present before the age of 20 years in 25% to 30% of all patients with inflammatory bowel disease. Reported incidence figures vary considerably depending on the collection of data. Multicenter, multinational collaboration is needed when studying pediatric inflammatory bowel disease. The essential first step is uniformity in the work-up and criteria used for diagnosis. The Porto diagnostic criteria presented here provide the tool that is needed. These criteria are the result of consensus reached by the ESPGHAN inflammatory bowel disease working group. Diagnosis of Crohn disease, ulcerative colitis and indeterminate colitis is based on clinical signs and symptoms, endoscopy and histology and radiology. Every child suspected of inflammatory bowel disease should undergo a complete diagnostic program consisting of colonoscopy with ileal intubation, upper gastrointestinal endoscopy and (in all cases except in definite ulcerative colitis) radiologic contrast imaging of the small bowel. Multiple biopsies from all segments of the gastrointestinal tract are needed for a complete histologic evaluation. A diagnosis of indeterminate colitis cannot be made unless a full diagnostic program has been performed.