Over the last two decades many countries and regions throughout the world have taken steps to establish Spatial Data Infrastructures (SDIs). Developing SDIs requires a considerable amount of time, energy and financial resources. Therefore it is increasingly important to assess SDI outcomes in order to justify the resources spent on those infrastructures. The main objective of this thesis is to develop a framework for assessing Spatial Data Infrastructures. This thesis achieves this main objective in four sub objectives. First, for the reason that SDI complexity is seen as a serious obstacle for its comprehensive assessment, this thesis analyses SDI complexity by determining whether SDI can be viewed as Complex Adaptive System (CAS). Second, the principles of assessing CAS and the norms of general evaluation theory serve as the basis for developing a Multi-view SDI assessment framework. Third, the thesis evaluates the application process and applicability of the Multi-view SDI assessment framework to assess SDIs by applying it in 21 National SDIs. Fourth, the thesis expands the Multi-view SDI assessment framework by adding an assessment view for a goal-oriented SDI assessment. The thesis results show that SDI can be viewed as Complex Adaptive System (CAS). The principles of assessing CAS serve as a basis for developing a Multi-view SDI assessment framework which allows for a more comprehensive and less biased SDI assessment. The evaluation of the Multi-view SDI assessment framework in 21 National SDIs demonstrates it potential to assess SDIs. This thesis also expands the Multi-view SDI assessment framework by adding an assessment view for a goal-oriented SDI assessment. The developed assessment view is tested by applying it to measure the goals realization of the Dutch SDI.