The object of this book is to supply elementary knowledge of metallic materials used in the construction and operation of machines. The aim has been to bring together a group of correlated facts, to state them clearly and to show their relation. The book is in two parts: the first part deals with the manufacture of materials; and the second deals with the properties of materials. Understanding of the first part is believed to be an essential preliminary to the study of the second part, while understanding of the second part is very desirable for those who design, construct and operate machines. The writer wishes to express his great obligation to Professor Herman Diederichs for his ever-ready and efficient help on part first, to Professor George B. Upton whose researches and kind suggestions made it possible to present part second in its present form, and to Professor Heinrich Ries for his kindness in criticizing the chapter on Refractory Materials.