This book is basically designed for university use, it will serve as a worthy reference for pure mathematics researchers particularly on structures of unit groups of commutative rings. The approach used to solve problems in this book is relatively straight forward and only requires the very basic knowledge of abstract algebra. Previous studies have restricted the classes of groups or rings to be considered. The determination of the structures of the unit groups of both Galois and completely primary finite rings has been of significant interest in the recent past.