In the recent years Magnetic Resonance Imaging has evolved into one of the most powerfuldiagnostic techniques in medicine, in part thanks to the application of suitable contrastagents. The design of new, more efficient MRI contrast media requires the complete under-standing of all factors and mechanisms that influence proton relaxivity, hence efficiency ofGd(III) complexes. In this chapter we give an overview of our current knowledge in this fieldby shortly surveying theory and citing the most illustrative examples. We discuss each of theunderlying factors, including the possible ways of their determination. Most recent develop-ments in the field of electron spin relaxation and outer sphere relaxivity have also beenreviewed. The last part is devoted to the comparison of the isoelectronic Eu(II) and Gd(III)complexes, which can give insight into the relaxation mechanisms of paramagnetic lan-thanides in general, and thus help design novel Gd(III) based agents.