Publisher Summary This chapter discusses tribology of the tool–chip and tool–workpiece interfaces—namely, (1) the distribution of contact stresses, (2)nature and interdependence of contact processes, (3) correlations between characteristics of the contact process with important input and output parameters, and (4) the characteristics of the cutting process. The tool–chip and tool–workpiece interfaces are those contact areas that directly participate in the cutting process. The different functions of the tool–chip and tool–workpiece interfaces in this process define the differences in the tribological processes at these interfaces. Although there are a number of differences, there are also a number of similarities in the tribology of these interfaces, such as high contact pressures and temperatures, contact with freshly formed (juvenile) counter-surfaces, and high sliding velocities. There is also a correlation between the stresses at these interfaces as they both are related to the same state of stress in the deformation zone. As such, however, the tool–chip interface plays a leading role that defines to a large extent this state. Therefore, a greater part of the considerations presented in this chapter is dedicated to the tribological conditions at the tool–chip interface.