This introductory Article provides a conceptual umbrella for the Special Issue on Informal Institutions and Democratic Decay. It offers conceptual clarity to studying informal institutions and explains their relationship to other concepts such as constitutional conventions or judicial culture. The article summarizes findings of the Special issue in four key observations. First, it shows that it is impossible to understand the functioning of courts without understanding the informal rules that shape courts’ governance and decision-making. These informal rules (institutions) appear within courts (internal), between courts and other actors (mixed) and among non-judicial actors with effects on courts (external judicial institutions). Second, it identifies a strong trend of formalization of rules, sponsored mostly on the supranational European level .Third, it explains why reforms of formal rules are often not sufficient to trigger behavioral changes and highlights the role of informal institutions in created commitment of actors to key democratic principles. Fourth, it argues that informal judicial institutions significantly impact the quality of democracy.