The transition between the superconducting and normal states is of extreme practical importance because the very sharp onset of resistance in voltage biased thin films is the basis for transition-edge sensors (TESs). TESs are being successfully utilized in many new instruments despite the fact that there is no consensus model that describes the resistance as a function of both temperature and current R(T ,I). A new model assuming a TES can be described as a resistively shunted junction (RSJ) has generated much interest. Here we compare the predictions of this model with the predictions of a two-fluid model and measured data. Except for some small TESs (characteristic size <125 μm), the data are not consistent with the RSJ model but are consistent with the two-fluid model.