Irritable bowel syndrome (IBS) is a chronic disorder with symptoms that range in intensity from mild and infrequent to severe and continuous. A variety of approaches to symptom assessment are used in IBS, although there is little literature directly comparing or validating them. Determining the presence or absence of specific symptoms is the primary focus of diagnostic evaluation and categorization. In contrast, outcome assessment usually entails assessment of symptom severity. Symptom severity scales can themselves vary on a wide range of factors, including specificity (pain, discomfort, gastrointestinal problem), scaling properties (numerical, analogue, or descriptor), range (usual, highest, lowest), time frame (now, past week), response category (intensity, unpleasantness, change, relief), and use of modifying variables (frequency, impact, location). The measurement properties of similar symptom scales have been investigated more extensively in the context of somatic pain, and these studies will be reviewed in suggesting some guidelines for IBS symptom assessment in clinical trials.