Contrast medium is a substance, which is used to image organs and fluids within the body cavity after introduction of contrast into the body cavity or vessels to help visualize the structures or pathology. Types of contrast media, properties of ideal contrast media, side effects and reactions, prevention of reactions, treatment of contrast-mediated adverse reaction, contrast agents in computed tomography (sometimes also called dyes), and magnetic resonance imaging contrast agents are also discussed in this chapter. Computed tomography contrast agents are used to enhance specific lesions in an organ, tissues or in blood vessels. In this chapter, multiple choice questions are provided along with their answers.