This keynote introduces the concepts behind and principles of the performance assessment of national spatial data infrastructures (NSDIs). In addition, it focuses strongly on four assessment approaches; SDI-Readiness, Clearinghouse Suitability, SDI-Organization, and INSPIRE/NSDI State of Play. These approaches are applied to assess the NSDIs of Croatia, Macedonia and Slovenia, and their assessment results are compared with other NSDIs in the world. The keynote ends with an overview of the main SDI-trends and some concluding remarks about the application of NSDI performance assessments.