The present understanding of the basic operational parameters of two- and three-terminal metal-thin insulator-semiconductor-semiconductor (MISS) switches is reviewed briefly. Although several different approaches have been tried, since the first descriptions of MISS devices in 1972, agreement between theory and experiment is at best semiquantitative. Even a parameter as basic as the (dc) static threshold voltage is not accurately predicted by current models. In the present paper a thyristor analogy is developed more fully and quantitatively, drawing upon well-established theories of instabilities in p-n-p-n structures. By making the justified assumption that in MISS devices inversion at the semiconductor-insulator interface forms only when the switching voltage is attained, it is possible to predict the threshold voltage and its temperature dependence in two-terminal structures. The effect of base current on the switching voltage of three-terminal devices can similarly be accounted for in a phenomenological way. Comparisons are made with published data from several sources and the thyristor model is shown to provide good agreement between experimental data and calculated parameters (values of the threshold voltage) for MISS devices with silicon oxide insulating layers.