The introduction of Enterprise Identity Management Systems (EIdMS) in organizations even beyond the purely technological level is a costly and challenging endeavor. However, for decision makers it seems difficult to fully understand the impacts and opportunities arising from the introduction of EIdMS. This book explores the relevant aspects for an ex-ante evaluation of EIdMS. Therefore it examines this domain by employing a qualitative expert interview study to better understand the nature of EIdMS, as they are situated between security and productive IT systems. To this regard, the focus is put on the general nature of EIdMS projects and the constructs being relevant for analyzing such projects in the decision support phase. Based on the derived constructs and thematic topics from the interviews, an explanatory model for EIdMS introductions is derived and iteratively improved and evaluated. Finally, a possible application use-case for the creation of adequate decision support tools is presented.