Political scientists developed the concept of informal political institutions to refer to an institution operating outside of the formal rules established by a political constitution and its auxiliary legislation. Among the many political behaviors that can escape formal regulations, however, only some of them conform a political institution. We devote the following discussion to identify their characteristics. First, regarding the political field, we discuss the concept of informal political institutions based on a review of North’s definition of institution, a condition to understand political institutions in general as well as informal political institutions. In a second part, we analyze the relationships between formal and informal institutions and illustrate some of its applications and functions in political systems. In the third and last part, we discuss the main methods for studying informal political institutions, as well as the implications and drawbacks of each methodological approach.