This chapter presents the basic definitions and visualisations of the major components of the cutting tool geometry important in the consideration of the machining process. The types and properties of modern tool materials are considered as well, as a closely related topic, as these properties define to a great extent the limitations on tool geometry. The basic mechanisms of tool wear are discussed. Criteria and measures of tool life are also considered in terms of Taylor’s tool life models as well as in terms of modern tool life assessments for cutting tools used on computer numerical control (CNC) machines, manufacturing cells and production lines.