The study of electron spin relaxation in aqueous Gd(III) complexes is the source of new insights into the physics and chemistry of magnetic resonance imaging (MRI) contrast agents. The coupling of the seven unpaired electrons of the Gd(III) ion with the surrounding water protons observed in MRI is the basis of the contrast agent effectiveness. Therefore, understanding the behavior of the electron spin system can provide valuable information for the development of new compounds. The availability of high frequency electron paramagnetic resonance (HF EPR) spectrometers is vital for complete relaxation studies, and played an important role in improving our knowledge of Gd(III) electron spin relaxation in the last few years. Variable temperature HF EPR has been an invaluable tool to improve our understanding of the underlying relaxation mechanisms.