This book sums up the results of more than 50 years’ work on the physico-chemical characteristics and biological activities of dinitrosyl iron complexes (DNIC). The interest of researchers in DNIC increased dramatically after the discovery of the fundamental role of the DNIC constituent nitrogen monoxide (NO) as one of the most universal regulators of metabolic processes occurring in living organisms. By virtue of their ability to donate NO and nitrosonium ions (NO+), DNIC mimic their beneficial (regulatory) and detrimental (cytotoxic) effects on living organisms and represent their working form.This text will appeal to practitioners in the field of biology, and other areas of modern-day science.