Diffusion-weighted magnetic resonance (MR) imaging has emerged as an important tool in the diagnostic work-up of patients with bowel cancer and inflammatory conditions of the gastrointestinal tract. It functions on the basis of the microscopic motion of water molecules in a cellular environment and provides functional information about the water in body tissues. Diffusion-weighted imaging serves to complement conventional MR imaging, and its use may improve the accuracy of tumor detection and staging. It does not rely on the use of intravenous contrast material and may be performed in patients with renal impairment. Because it provides quantitative information about tissue cellularity, diffusion-weighted imaging may be used to distinguish between tissues with altered cellularity (eg, tumors and metastases) and normal tissues. Data from diffusion-weighted MR images enable the calculation of apparent diffusion coefficient (ADC) values, which provide useful information about response to treatment. Malignant gastrointestinal tract tumors have low ADC values, which increase after successful therapy. Diffusion-weighted imaging also plays a role in the evaluation of patients with inflammatory bowel disease and may help assess inflammation and complications, such as abscesses and fistulas. Quantitative measurements of signal intensity at diffusion-weighted imaging may help differentiate actively inflamed bowel from normal bowel, and ADC values provide useful information about disease activity and response to treatment.