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A brief survey is presented of the mechanisms which have been proposed for the erosion of ductile metals by solid particles. After reviewing these and examining scanning electron microscope photographs, it is concluded that a ductile cutting mode applies when the velocity vector of the eroding particle makes an angle of less than about 45 deg with the surface. Above this angle the removal process appears to involve quite different mechanisms. An earlier analysis of the cutting mechanism is reexamined and shown to predict many features of the erosion process. In particular, the roles of particle velocity, elevated temperatures, and material properties are discussed. Some preliminary results are presented for erosion at higher angles, and possible mechanisms for material removal are discussed. Finally, some suggestions are made for future directions in erosion research in view of the current interest in coal-hydrogenation processes.