The following descriptions and explanations were written mainly for the use of those actively engaged or interested in the commercial production of case-hardened objects. For that reason the chapters are arranged so as to appeal at once to the workshop experience and observations of craftsmen, to whose friendliness the author is indebted for many of the specimens from which the illustrations were made. It was not found possible, however, to separate the subject into practical and theoretical divisions, nor is any such distinction desirable. Though no apology may be needed for introducing a number of micro-photographs, a few words of explanation may be allowable. We are accustomed to discriminate between certain kinds of materials by the appearance of fractured surfaces; the tool steel trade was built up on refined discriminations of this kind long before chemical analysis or the modern refinements of heat treatment had been developed to any serviceable extent. The observation of polished and etched surfaces is nothing more than an extension of this old and useful practice, and quite a remarkable amount of information can be extracted from such surfaces by means of a hand lens magnifying only five or six diameters, and sometimes even by the unaided eye.