The occurrence of IBD has been increasing in Western Europe and North America over several decades after the Second World War. It has been assumed that socioeconomic factors represent the most important explanation for this increase . A part of this increase may have been related to more awareness and recognition of the diseases, as well as generally better registration in all countries. It is important to realize that the general acceptance of endoscopy as the main internationally accepted diagnostic procedure is quite young, and that we may divide the history into a pre-endoscopic area before 1970, an early endoscopic period between 1970 and 1990 characterized by a relatively large heterogeneity among studies, and a post-endoscopic period from around 1990, whereafter a widespread distribution of equipment and skills of endoscopy enabled all countries to perform uniform diagnostic procedures (Table 1.1). Moreover, after this point in time, most international studies have been performed according to generally accepted definitions and criteria of diagnosis. However, even if we generally include only endoscopy-based studies, the heterogeneity of even the Western materials is striking and difficult to compare, regarding incidence, prevalence, and subtypes. One important reason for this is the selection of cohorts in the different countries. In most centers, the registration of IBD has been hospital based, by which the type of recorded patients were depending on the level of each hospital in the healthcare system of each country, including access to health care. Additionally, great variations exist in the recording systems, both between hospitals and between countries, and in how well the patients were characterized on the basis of first or later admissions .