A full understanding of the presentation and work-up of inflammatory bowel disease is necessary to ensure appropriate treatment of this complex disease. Crohn's disease and ulcerative colitis share many common clinical features but are treated very differently. This article covers the factors which contribute to IBD pathogenesis and presentation as well as the methods of diagnosis and work-up to ensure that the appropriate diagnosis is reached. This article also serves as a basis of understanding for the more complex aspects of the disease to be discussed in subsequent articles. Copyright © 2019 Elsevier Inc. All rights reserved.