A theoretical analysis is presented for the erosion of metals by spheres at normal incidence. The model employs a criterion of critical plastic strain to determine when material will be removed, and predicts velocity exponents of 3 for erosion and -2 for the mass of spherical particles which must hit the surface before material is removed. The mechanical properties of the metal are described by two quantities: its dynamic hardness and its ductility under erosion conditions. Data obtained in experiments with aluminium alloys as well as previously published data are compared with the theory.