The importance of inter-organizational system (IOS) has been increasingly recognized by organizations. However, IOS adoption has proved to be difficult and, at this stage, why this is so is not fully uncovered. Based on a previously published empirical study, the authors argue that the ihfactorly approach, which has been widely used to study technology adoption, is not generally appropriate to study IOS adoption. To further examine this claim, this paper reviews the literature in order to identify the current state of IOS adoption studies. For this purpose, the factor and processual approaches are defined and used to bring order to the existing studies. This review finds that, until recently the majority of IOS studies have used the factor approach and that they have yielded inconsistent results. It also demonstrates that the use of the processual approach is just being recognized and a research gap exists for this kind of study.