The object of this course is to outline the way in which the strength of metals, at atmospheric temperature, varies with their microstructure. Five groups of specimens specially prepared under controlled conditions are provided for hardness determinations and microscopical examination. This book describes the principles involved, gives full practical instructions, and continuous commentaries are coupled with typical results on each specimen sequence. The specimens have been chosen to demonstrate the various factors that control the engineering strength of a metal, and to show how improvements are made in practice. The following effects are illustrated: grain size, cold work, solid-solution hardening, compound formation, quench hardening and tempering, and precipitation hardening. Where appropriate, the quantitative aspect of microstructure is emphasized. Thus, in addition to grain size, the question of grain boundary surface area, together with the determination of volume fraction and distribution of second phases are discussed. Hardness is taken as a convenient index of strength from the point of view of ease of specimen preparation and property determination. It is true that hardness as conventionally determined by indentation methods is not a simple characteristic, but involves a combination of elastic and plastic behavior. However, in a given series of related materials, the results provide a satisfactory basis for comparison, and the more specific strength terms generally follow the same pattern. To support the hardness results, typical tensile figures for the materials are quoted in the text as well as notched-bar impact values where appropriate. Wrought materials are used for the specimens because they are sounder and more uniform than cast metal, and thus better suited for hardness measurements. The exercise is meant to be elementary in that theoretical ideas are described in their simplest form, and moreover, the general principles are established with respect to the selected examples. The present text is written to be complete in itself, but the companion volume, entitled ‘The Role of Microstructures in Metals,’ gives a broader and profusely illustrate treatment of the subject matter, particularly in reference to the nature and development of microstructure. The material following is presented under four major headings. First, after a brief summary of structural details, the effects of stress on metals are outlined so that the principles of strengthening may be understood and the main strength terms defined. Secondly, general recommendations on practical procedure are given including a section on microscopical measurements, that is, quantitative metallography. Thirdly, specific instructions and typical results are provided for the selected specimens, together with a self-contained and continuous commentary on the principles involved. Lastly, a brief consideration is made in general terms of the order of magnitude of conventional strengthening mechanisms. Although the emphasis in on the strength of metals under conditions of slow loading at atmospheric temperature; for the sake of completeness, this last section concludes with a brief indication of the significance of microstructure in respect of brittle fracture, fatigue, stress-corrosion, and creep. It is thought that, on one hand, the course provides useful introductory material for those approaching the subject with an engineering bias. At an early level, the specimens represent a coherent series for simple testing work, rather than using random examples. In such cases, it is not necessary to make the microscopical examination, as the text and illustrations supply a background of information which should illuminate the practical work. On the other hand, for those with more of a physical or metallurgical interest, the course offers an easy starting point for work on microstructures without involving any great knowledge of transformations in metals. At the same time, the exercise has practical significance, for real engineering materials are involved and interest is not narrowed only to structure, as this is continuously correlated with mechanical properties. In another context, that of materials science, the course provides a valuable exercise on the relations between properties and the structural morphology of crystalline aggregates, applied to materials that have been rigorously studied, and that are easy to handle and prepare.