The aim of using a contrast agent in Magnetic Resonance Imaging is to accelerate the relaxation of water protons in the surrounding tissue. Gadolinium (Gd)(III) complexes are far the most widely used magnetic resonance imaging (MRI) contrast agents in the clinical practice. This chapter focuses on the discussion of relaxation theory and the experimental results on Gd(III)‐based agents. It first discusses the inner‐sphere proton relaxivity by analyzing each factor that determines this relaxation mechanism. This will be followed by a survey of the second‐ and outer‐sphere contributions to proton relaxivity. The chapter also discusses in detail proton nuclear magnetic relaxation dispersion (NMRD), a technique that is widely used and rather specific for the characterization of MRI contrast agents. Finally, the chapter draws conclusions from the existing results and gives indications on how to optimize the parameters that govern proton relaxivity, with the aim of designing more efficient MRI contrast agents.