In the manufacturing industries, despite the development and improvement of metal forming processes, a great deal of reliance is still placed on metal cutting processes and this will continue into the foreseeable future. Thus, there will continue to be a requirement for the development of improved cutting tool materials, workpiece materials, cutting fluids and testing methods; collectively this activity can be described as improving machinability. Machinability is a parameter which in many ways is vague, sometimes qualitative and very often misunderstood. The purpose of this text is to give a broad understanding of the concept, methods of assessment and ways of improving machinability to the manufacturing engineer, the metallurgist and the materials scientist. The text should also be of interest to those engaged in research in manufacturing engineering and metal cutting. The text, of necessity, does not attempt to give detailed information about the machining characteristics of a wide range of tool and workpiece materials. It is felt that this is beyond the scope of the book and is best left to other sources, such as machinability data banks and the Machining Handbook*, whose main objective is to present this kind of information. It is hoped that the reader will be able to progress logically from the fundamental aspects of the metal cutting process to the sections on the more specific topics of machinability including machinability testing and the properties of tool and workpiece materials which affect their machining performance.