Crohn disease is a chronic inflammatory disorder of the intestines that is characterized by multiple episodes of exacerbation and remission. Although barium studies and endoscopy remain the most valuable tools for assessment of mucosal and luminal changes, sonography has proved to be a safe and noninvasive modality for characterization of mural and perienteric changes. Ultrasonography (US) can accurately demonstrate the classic features of Crohn disease as well as the complications frequently associated with the disease. The addition of color Doppler imaging may allow differentiation of chronic from active bowel wall thickening. In expert hands, US appears to be a sensitive modality for preliminary investigation of patients with symptomatic Crohn disease. US can also be used to assess the response to treatment and to detect postoperative recurrence. Finally, US enables accurate selection of patients in whom the possibility of surgery or percutaneous intervention would justify other means of investigation, particularly computed tomography.