Organizational information systems (IS) must be used effectively to maximize their benefits for the overall organization. However, their complexity creates significant challenges for conducting the IS project of an implementation in such a way that users are subsequently enabled to use a system effec-tively. The challenge is also present in the post-implementation phase whenever employees have to learn how to use a system effectively for a given task and develop their own adaptations. Thus, this thesis presents a research effort on the means that effect users’ perception and ability to use a system effec-tively during an IS project as well as in the phase of use in post-implementation. Thereby this thesis provides a more detailed understanding of the concept of effective use and the conditions that enable effective use of IS. The analysis of IS projects contributes to research on the ambiguous and subjective nature of the evaluation of IS project success and explores the configuration of user involvement and participation in implementation projects that is most likely to be related to project success. The results of the analysis show that the participation of the appropriate users in the requirements analysis phase is most often related with IS users that feel enabled to use an IS effectively. ... mehrResearch on the IS use in the post-implementation phase presented in this thesis provides more detailed conceptual development of the relationship of user behaviors such as learning, user adaptation, and workarounds with effective use and thereby extents the nomological net of theory of effective use (Burton-Jones & Grange, 2013). Results of further analysis show that workarounds can positively affect the use of an implemented IS. Finally, this thesis contributes an operationalization of the concept of effective use based on the initial conceptualization by Burton-Jones and Grange (2013). This answers a call for research on richer concep-tualizations of IS use. In sum, these findings add to research on concept of IS use and user behaviors that influence the effective use of IS. Therefore, they enable a more detailed understanding of the concept of effective use and allow future researchers to extent and refine the nomological net of effec-tive use. Practitioners may also employ these new measures to measure the effectiveness of their efforts to improve the effective use of IS. They also benefit from the insights on the perception of IS success in organization and appropriate phases and forms of user participation in IS projects.