Magnetic Resonance Imaging (MRI) is an important diagnostic tool. One third of MRI procedures on patients use contrast agents, which are administered to improve image quality. Eight nanoscale contrast agents, in development, are presented and compared. They range in size from 3nm to 200 nm and they represent the latest advances. They are paramagnetic, gadolinium based or superparamagnetic, iron based. Some of the contrast agents can target specific ions or diseased cells. Their relaxivities are better than the present clinical standards but they have not yet been tested in humans. They offer new possibilities for the diagnosis of diseases like cancer.