Constitutional democracy—representative democracy founded in a constitution and recognition of fundamental rights—is a fragile form of governance that can slowly disappear without protection, support and maintenance. What can make constitutional democracy more resilient and prevent it from turning into a form of illiberal democracy or authoritarian rule? Within legal and political theory, this question has received considerable attention from the 1930s onwards, leading to numerous studies on militant democracy. These studies, however, mainly focus on one of the two pillars of constitutional democracy, democracy, while the other pillar, constitutionalism and rule of law, has received far less attention. With some minor exceptions hardly any articles or books are dedicated to the question what the concept of militant constitutionalism entails (i) or what a legal theory of militant constitutionalism should look like (ii). Those publications that do address the concept of militant constitutionalism, moreover, primarily focus on formal instruments of constitutional defense (rule of law institutions) and not on informal instruments (norms, ethos, practices and culture). Yet, what kind of informal instruments or legal culture are needed to maintain and defend constitutional democracy (iii)? This chapter has two main goals. The first is to answer question (i) by mapping different understandings of the concept of militant constitutionalism. The second goal is to give a general response to question (iii) by distinguishing four possible elements of such a legal culture.