Studies of constitutions in authoritarian regimes reveal a new finding hiding in plain sight: that the military is often a key constitutional actor. The question of how the military uses law and constitutions to enable and facilitate its influence in constitution making and constitutional practice is under-researched. The military demands scholarly attention because of the unprecedented opportunities for the military in governance due to the rise of populism and the decline of democracy, internal conflict, efforts at counter-terrorism and anti-trafficking, and the COVID-19 global pandemic. I review the literature across law and the social sciences on the constitution and the military in authoritarian regimes. In doing so, I demonstrate that the military is an important, yet overlooked, constitutional actor; that civilian control of the military by law is never absolute but a matter of degree and changes over time; and that histories of military rule and military use of law and constitutions matter.