Underspecified performance requirements can cause performance issues in a software system. However, a complete, upfront analysis of a software system is difficult, and usually not desirable. We propose an evolutionary model for performance requirements specifications and corresponding validation testing. The principles of the model can be integrated into agile development methods. Using this approach, the performance requirements and test cases can be specified incrementally, without big upfront analysis. We also provide a post hoc examination of a development effort at IBMthat had a high focus on performance requirements. The examination indicates that our evolutionary model can be used to specify performance requirements such that the level of detail is commensurate with the nature of the project. Additionally, the IBM experience indicates that test driven development-type validation testing corresponding to the model can be used to determine if performance objectives have been met.