Paramagnetic metals can induce T1 shortening by interaction with free water molecules. Two metal ions, Gadolinium and Manganese, are currently available for human use. Gadolinium-based MRI contrast agents (CAs) can operate using a approximately 100-fold lower concentration of Gadolinium ions in comparison to the necessary concentration of Iodine atoms employed in CT imaging in the tissues. Therefore, numerous macromolecular MRI CAs prepared employing relatively simple chemistry are readily available that can provide sufficient enhancement for multiple applications. Herein, we describe the synthesis, characteristics, and potential applications of dendrimer-based macromolecular MRI CAs in our recently reported libraries. This entire series of dendrimer-based macromolecular MRI CAs have a spherical shape and possess similar surface charges. Changes in molecular size altered the route of excretion. Smaller sized contrast agents, of less than 60 kD molecular weight, were excreted through the kidney resulting in these agents being potentially suitable as functional renal contrast agents. Less hydrophilic and larger sized contrast agents were found better suited for use as blood pool contrast agents. Hydrophobic variants of CAs formed with polypropylenimine diaminobutane dendrimer cores quickly accumulated in the liver and can function as liver contrast agents. Larger hydrophilic agents are also useful for lymphatic imaging. Finally, contrast agents conjugated with either monoclonal antibodies or with avidin are able to function as tumor-specific contrast agents and might also be employed as therapeutic drugs for either gadolinium neutron capture therapy or in conjunction with radioimmunotherapy.