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; also practice and artifacts are theory-laden. The concept of knowledge organization system (KOS) is briefly introduced and discussed. A theory is a fundamental form of KOS and theories are the point of departure of any KOS. It is generally understood in KO that concepts are the units of KOS, but the theory-dependence of concepts brings theories to the forefront in analyzing concepts and KOS. The study of theories should therefore be given a high priority within KO concerning the construction and evaluation of KOS.