Inflammatory Bowel Disease (IBD) can be classified into two main subtypes: Crohn’s Disease (CD) and Ulcerative Colitis (UC). The aim of this study is to identify the genetic contribution to the susceptibility to IBD. In the first part of the study, I focused on Crohn’s Disease, the subtype that shows the greatest heritability. Using both pooled and sub-phenotype data, followed by replication, the results reveal substantial genetic heterogeneity and the total number of confirmed CD susceptibility loci was increased from 71 published by others to 200. This was achieved by analyzing the data using a multimarker approach and high-resolution genetic maps in Linkage Disequilibrium Units. In the second part of the study, I focused on Ulcerative Colitis. The results show that although UC has a lower reported heritability, many loci were also found for Ulcerative Colitis. Some of these overlap with those found for Crohn’s Disease.