Workers value job security. If at least some workers value it enough, then it is efficient for at least some firms to adopt policies in which they commit (implicitly or explicitly) not to dismiss employees except for "just-cause," as opposed to policies in which employers are free to dismiss employees "at-will." In this paper, we develop a simple model in which the equilibrium distribution of workers between just-cause firms and at-will firms is not generally efficient: there can be inefficiently many workers in just-cause firms or inefficiently few. If there are inefficiently few, then a tax or even a ban on at-will firms can be welfare-improving.