The aim of this paper is to explain the concept of the informal within organizations. We deploy two complementary theories: organization theory and institutional theory. In spite of undeniable contributions, the concept of the informal remains inevitably linked with different concepts such as informal organization or informal institution. Little research considers the informal as a theoretical object. Thus we try to theorize the informal and to understand this controversial concept. We conclude that, because of its importance and interdisciplinarity, the informal must be seen as a meta-concept in the study of organizations.