Because contemporary treatment of oral cavity cancer involves procedures that spare the tongue and mandible, an adequate assessment of the oral cavity is essential for appropriate surgical and radiation therapy planning. Computed tomography (CT) and magnetic resonance (MR) imaging, which allow differentiation between soft tissues, are valuable tools for assessing this complex region. Their main advantage resides in their capacity to show at best the normal anatomy and the exact extent of a low-lying tumor. For display of soft tissues and tumor, MR imaging, being a multiplanar and multicontrast technique, is superior to CT. Nonenhanced T1-weighted MR imaging is better for defining the exact extent of medullary bone invasion, which appears as a low-signal-intensity area within hyperintense medullary fat. CT is optimal in detection of cortical bone invasion, which appears as an interruption or erosion of the peripheral hyperattenuating rim. Thus, in cancer of the tongue, MR imaging should be performed first. If tumor extension to the mandible is suspected (due to clinical or MR imaging findings), CT should be added. In cancer of the floor of the mouth, both MR imaging and CT should be performed in the initial work-up, especially in those cases in which there is a clinical doubt about mandibular extension of disease. The main drawback of both modalities is their lack of specificity; other methods are needed to discriminate between tumors and inflammatory or infectious diseases, particularly in the mandible. However, once the diagnosis has been confirmed histologically, treatment can be chosen based on complementary information obtained from CT and MR imaging.