The paper gives a brief critical review of some of the more important contributions to this subject. All of these develop methods of calculating surface losses which are largely empirical and give no clear picture of the flux-density variations in the air-gap, due to the presence of slot-openings, which are the sources of surface losses.The paper applies Carter's field equations to the determination of the wave shape and amplitude of the flux ripples produced by the slot openings. The fundamentals of these non-sinusoidal ripples are resolved into rotating flux-density fields so that the losses produced by these harmonic fields in the iron and winding circuits traversed by them may be calculated.It is further shown that the currents produced in a squirrel-cage rotor by the stator slot-openings are at stator slot frequency and that these currents produce rotating fields whose frequency at the stator is a function of the number of rotor slots.The list of references includes a representative selection of the papers published on the subject.