The transfer of momentum between a conduction electron and a crystal lattice is examined in detail. By physical considerations we are led to the expression(mm*)ij = δij+∑GGiδδkj|aG(k)|2for the reciprocal effective mass tensor, whereGis2πtimes a reciprocal lattice vector andaG(k)is a coefficient in the momentum representation of the Bloch eigenfunction:ψk = ∑GaG(k) exp[i(G+k)·r]. This expression for the effective mass tensor is shown to be completely equivalent to the usual definition.