Modern American presidents tend to want more power, so they often latch onto to theoretical accounts that argue for enhanced presidential power, as the unitary executive theory does. This chapter examines how the theory of the unitary executive has developed, from its founding under Ronald Reagan to reverse the post-Watergate decline in presidential efficacy and to enhance presidential control of the bureaucracy, to its further development under George W. Bush to permit unilateral presidential control of foreign policy in the “war on terror,” to its (mis)use under Donald Trump to justify actions intended to aid his personal interests, rather than those of the presidency or the country. We argue that the unitary executive theory has changed significantly from its original purposes, and in the current political environment it poses real dangers to the well-being of America’s constitutional democracy.