This book is an attempt to explain hardness measurements of metals in terms of some of their more basic physical properties. It does not deal with the atomic and crystalline mechanism of elastic or plastic deformation, but starting with the elastic and plastic characteristics it shows that the hardness behavior of metals may be expressed in terms of these characteristics. It is hoped that this presentation will provide, for physicists, engineers, and metallurgists, a better understanding of what hardness means and what hardness measurements imply. The book places an emphasis on the physical concepts involved, so that the non-mathematical reader may grasp and appreciate the general physical picture without needing to follow the more detailed treatment.