Ulcerative colitis is a chronic inflammatory bowel disease characterized by continuous mucosal inflammation of the large bowel. However, by conducting a literature search, it emerges that, although being considered a primary mucosal disorder in a subset of patients, the inflammatory process may extend beyond the mucosal surface. For this reason, we reviewed the pertinent literature to evaluate the evidence related to the aforementioned topic. The literature analysis confirmed that, although ulcerative colitis has to be defined as a primary mucosal disease due to its consistent mucosal onset, it can involve deeper layers of the colonic wall. The inefficacy of anti-inflammatory therapies in a considerable proportion of patients, along with the lack of histologic healing and the persistence of inflammatory status and colonic wall thickening at imaging despite mucosal healing, has led to consider an extension of the disease process beyond the mucosal layer. The recent application of more accurate diagnostic tools, both histological and radiological (i.e., intestinal ultrasound and magnetic resonance), has the potential to underline the early signs of disease extension and progression in order to improve ulcerative colitis clinical management.