Diagnostic procedures are used throughout the course of a patient’s journey with IBD for different reasons. Initially, testing is used to establish a diagnosis, assess severity and the risk of complications and progression. Once therapy is established, some procedures are required to assess the response to treatment in order to optimise the patient’s health and long-term prognosis. Frequent testing of blood is required to monitor for potential complications from either the disease or some of the therapies used to treat it. This section will outline common diagnostic tests and reasons for their use.