Since the development of the first clinical MRI contrast agent, Gd(DTPA) (or Magnevist®) in the early 1980s, another three linear and three macrocyclic (eg. Gd(DOTA) (Dotarem®) have been introduced in the clinic and inspired a huge number of researchers worldwide to perform preclinical research to obtain contrast agents for many kinds of clinical applications, which are described in this book. The most current research has been assembled here and can be used by the scientific community to continue increasing the relevance of metal-containing molecules and nanomaterials for MRI-based diagnosis and thus increase the practical use of these systems.