This book provides a unified understanding of metals and their alloys from the standpoint of chemical bonding theory. This book comprises two parts, Fundamentals and Applications. In the Fundamentals part, four theoretical quantities (cohesive energy, energy fluctuation, atomic population, and atomic bonding population) obtained from molecular orbital calculations are utilized to develop a unified logic for various problems about the relationship between bonding, structure, properties, and reactions of materials. In the Applications part, various practical metallic alloys (fifteen alloys such as Al, Fe, Ti, Cu, Mg alloys) are systematically organized into six alloy types (solid solution hardening type, age hardening type, work hardening type, corrosion resistance type, heat resistance type, casting type) and solutions to some disputed issues are proposed based on the chemical bonding theory. In particular, this book proposes a new concept of the strength of metallic materials instead of the conventional classical dislocation theory. This book intellectually stimulates young engineers/graduate students interested in materials science and professional experts involved in metallic materials by offering an innovative perspective for their research/development activities.