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Social scientists frequently refer to the concept of informal decision making. However, this concept is unclear because it can refer to very different aspects: the infringement of formal decision rules; decision practices that occur randomly and do not obey any rule; or the search for consensus. This article aims to deconstruct the concept of informal decision making and to offer theoretical tools in order to facilitate empirical research on cases of so-called informal decision making. In this respect, it will show that the neo-institutionalist theory provides us with helpful concepts. Furthermore, the analysis of different cases of informal decision making reveals reiterated decision practices and numerous unwritten rules with a complex relation to formal decision rules. Therefore, the concept of informality appears as not very helpful on the analytical and empirical levels.