This short chapter is intended to provide an overview of some of the most common commercially available magnetic resonance imaging (MRI) and x-ray/computed tomography (CT) contrast agents. Aside from the structure formula, active ingredient, and available concentrations, the reader will appreciate important biophysical information such as osmolality and viscosity as this is important for bolus injections. For MR contrast agents, the relaxivities are also listed. For more details on different contrast agents and their safety, the reader is referred to Chapters 7 and 8. Tables 37.1, 37.2, and 37.3 present the details for contrast agents for various testing modes.