Regucalcin is a protein which is highly differentiated due to increased conservation of the regucalcin genes throughout evolution in vertebrates. Regucalcin plays a multifunctional role in cellular regulation, including an inhibitory role in protein kinases, protein phosphatases, NO synthase, nuclear DNA and RNA synthesis. Regucalcin suppresses an enhancement of cell proliferation and apoptotic cell death that are induced by various signaling factors. Regucalcin plays a pathophysiological role in many metabolic disorders in humans. This book introduces recent topics of regucalcin studies concerning the genomics, cell regulation and diseases associated with this protein.