Magnetic resonance imaging (MRI) is a rapidly evolving technique that is becoming the imaging method of choice in small animal medicine because of its ability to produce superior, high-contrast, anatomically detailed images. Traditionally used for diagnosing diseases of the central nervous system, applications have broadened to include diagnosis of many common disorders, among them musculoskeletal injuries and diseases, nasal neoplasia, eye and orbital disease, and vascular malformations. More will probably be added. This article simplifies the science behind MRI and compares it with computed tomography (CT), radiography, and ultrasonography, highlighting the reasons for choosing MRI over other methods and challenging conventional wisdom. Technologic advances in both MRI and CT have made distinguishing the two techniques more subtle, with advantages and disadvantages in some cases related more to cost and availability than to image quality. Because MRI does not use radioactive isotopes, it virtually eliminates risk for local or systemic adverse reactions to contrast media. It documents the extent of disease more completely, which is usually borne out surgically or histologically. Radiography, while helpful, lacks the tissue specificity required for locating the origins of disease. Other advantages of MRI are the range of organ systems that can be imaged and the value of sequential magnetic resonance images in planning biopsy, surgery, and therapy. However, MRI is not considered appropriate or cost-effective in cases where radiography or ultrasonography would suffice. Indications for MRI use and the basic science of MRI are covered in detail and a continuing education test is included at the end of the article.