Package on Package (PoP) structures are an enabling technology for mobile devices, but they present unusual challenges in final test. Since the packaging technology is 3D, the test contactor must also take a multi-dimensional approach to the package. In final test applications a fairly standardized approach to the situation has been developed; while significant engineering challenges exist, methods for solving them are well known to top-tier socket manufacturers. System level and engineering characterization applications present much more diverse requirements, and require dynamic innovation in socketing. This paper describes the unusual requirements of contactors for final, system-level, and engineering characterization test in detail, and presents innovative solutions for resolving them.