This book covers the important aspects that vanadium complexes play in biological systems. Beginning with the synthesis and spectroscopic characterization of vanadium-hydrazide complexes, it goes on to describe the inhibition properties of these complexes on important enzymes like ureases, phosphodiesterases, alpha-glucosidases, and thymidine phosphorylases. These complexes exhibit interesting inhibition potential for these enzymes. Recognizing the role of free radicals in several diseases including diabetes and its complications, extensive studies of the interaction of these complexes with free radicals like DPPH, superoxide and nitric oxide are reported with a special emphasis on structure activity relationships. The complexes show promising inhibitory activity for all free radicals. Comprehensive coverage of bioactivities of vanadium complexes also includes leishmanicidal, fungicidal and bactericidal microbial studies.