Next generation Software Performance Engineering tools will exploit a model interoperability paradigm that uses the performance modeling tool best suited to the software/hardware architecture issues and the life cycle stage of the assessment. The paradigm allows the use of existing tools to the extent possible without requiring extensive changes to them. The performance model solution should be transparent to the user. Significant milestones have been accomplished in the evolution of this paradigm. This paper covers key results in the areas of Model Interchange Formats, model transformations, tools, specification of experiments and results, and extensions for real-time and component-based systems. It then offers conclusions on next steps and the future of the model interoperability paradigm.