This chapter reviews the role of diagnostic testing in functional gastrointestinal disorders (FGIDs), the importance of collaborative discussions with patient as to the role and describes expected outcomes of testing and how they relate to a holistic treatment approach. FGIDs comprise a heterogeneous group of conditions defined on the basis of symptomatology. A significant component of the health-care expenditure in FGIDs relates to the cost of investigation, and physicians who practise a greater degree of patient-centred communication have a lower diagnostic testing expenditure. As the symptoms of FGIDs are protean and commonly involve extraintestinal systems, testing needs to be tailored to the patient and to the symptom pattern; indeed if adequate previous investigations have been performed, further testing may not be required. Once diagnostic testing has been completed it is vital that the patient is informed of the result as soon as possible.