The notion “theory” is a neglected concept in the field of information science and knowledge organization (KO) as well as generally in philosophy and in many other fields, although there are exceptions from this general neglect (e.g., the so-called “theory theory” in cognitive psychology). This article introduces different conceptions of “theory” and argues that a theory is a statement or a conception, which may be questioned and which is connected with background assumptions. Theories form interconnected systems of grand, middle rank and micro theories