Magnetic Resonance Imaging (MRI) is a well-established diagnostic technique which allows images to be obtained of the inside of the human body. For several MRI procedures, to increase the quality and resolution of the images, a contrast agent is administered to the patient. Among the available MRI contrast agents, those based on the paramagnetic gadolinium ion are the most widely employed. The syntheses and applications of the MRI contrast agents currently available on the market are reviewed and discussed. Moreover, new perspectives in this field, such as responsive and bioactivating agents, macromolecular and supramolecular systems, and agents for innovative MRI approaches, are highlighted.