Manganese is an essential element. With five unpaired electrons, slow electron spin relaxation and fast water exchange, Mn2C is an excellent nuclear relaxation probe and the most promising alternative to replace Gd3C in contrast agents for magnetic resonance imaging (MRI). Given the labile nature of the Mn2C ion, its complexation in thermodynamically stable and kinetically inert complexes is difficult. Moreover, the complexes also need to contain inner sphere water to maintain good relaxivity. In the last decade, a large body of data has been gathered including linear and macrocyclic complexes as well as cage molecules which allow now to establish relationships between the structure of the Mn2C complexes and their stability, inertness and relaxation properties, and this constitutes the basis for a rational design of more stable and efficient probes. In addition, high spin Mn3C is also a good relaxation agent and Mn3C complexes, in particular porphyrins, have been investigated as MRI probes. Chelates of both Mn2C and Mn3C have been explored to derive smart MRI agents with a specific response to various biomarkers, such as pH, enzymes or redox indicators. This chapter surveys shortly these different fields where manganese-based complexes are exploited in relation to MRI.