This book proposes an innovative approach and an interpretation of the intrinsic rules and phenomena of metallic materials based on a quantum chemical standpoint. In this chapter, several well-known and practically important topics, such as solute solubility, work and age softening, castability of iron-carbon alloy, amorphous alloys, and hydrogen storage alloys, are chosen as familiar subjects and discussed in detail, based on a quantum chemical approach in Sects. 5.1 through 5.5. The authors unveiled a vast new scenery behind macroscopically visible phenomena and classic conventional interpretations by using quantum chemical estimations of cohesive energy, energy fluctuations, and Mulliken’s population parameters. In Sect. 5.6, the magnetic properties in the alloys are discussed. The magnetic properties materials should be treated as phenomena related to spin, and it must be said that non-magnetic approaches have their limitations. However, we believe that methods that do not take spin into account can still be approximately valid to a certain extent so as to consider the chemical contribution to magnetism. Hopefully, readers will understand that the present challenging approach aims to provide a wide range of applications and a profound scope to the researchers and graduate students involved in materials science and technologies (198).