Abstract A new theory of the i-v characteristics of superconducting weak links of the SNS sandwich type is outlined. The theory is based on a simple model current-density equation and a careful assessment of the internal impedances of the high- and low-frequency sources present. All the important differences between the characteristics for SNS sandwiches and tunnel junctions are explained at least qualitatively and, to a large extent, quantitatively. A characteristic feature of the theory is the phase-locking of the rate of change of phase across the junction to any applied high-frequency signal. This feature is especially important in understanding the appearance of current steps at the “sub-harmonic’ voltages ( n h ̷ ω/2 em), and in explaining the extreme steepness of the steps observed. The model is compared with the theories for other types of weak link.