A metal ion refers to a type of atom compound that possesses an electric charge. These ions are essential components for the maintenance of human beings, plants and animals. Their absence may result in serious malfunction, developmental disorders, carcinogenesis or death. Metal ions such as Mg2+, Ca2+ and Na1+ are essential cofactors in numerous cellular processes including cell proliferation, respiration, growth and immune function. They are also essential for the function of a large number of enzymes as structural or catalytic cofactors. Metal ions are extensively present in biological systems and take part in a variety of critical biochemical processes including energy conversion, metabolic regulation, material transportation and information transmission, which makes them an essential substance in the human body. This book aims to shed light on the role of metal ions in biological systems. A number of latest researches have been included to keep the readers up-to-date with the global concepts in this area of study.