Publisher Summary This chapter examines the operation and principles of both ultrasound imaging and magnetic resonance imaging (MRI). The different features and characteristics of the three imaging modalities, computed tomography (CT), MRI, and ultrasound have been examined in this chapter. The tissue properties being imaged are considerably different among the different modalities and can be complementary. Both CT and MRI can image the entire body with consistent resolution and contrast; however, they are the most expensive methods of imaging and are the least portable and slowest. Ultrasound, on the other hand, has variable resolution and penetration and limited access to certain portions of the body, and is used to image soft tissues only. However, it has advantages in low cost, portability, and speed. At the end of the chapter, applications of MRI including functional MRI (fMRI) is discussed and the main features of all major imaging modalities, including CT and x-ray is compared.