Model-based performance analysis is also known as Analysis by Synthesis, abbreviated to AnSyn. The method combines test analysis and performance prediction in a single program. The program calculates modifiers for the component models—the AnSyn factors—in a special test analysis mode, which makes the model agree exactly with the measured data. In this chapter, we describe how AnSyn is used in GasTurb across a range of engine configurations and degrees of instrumentation. We give an example of test analysis on a heavily instrumented development engine as well as a production engine returned to a maintenance shop with much fewer sensors. The chapter concludes with a real-world example of trend monitoring using the very limited instrumentation available during airline operations and this illustrates compressor deterioration after thousands of flight cycles. Model-based performance analysis is the industry standard during engine development and can identify small differences between components when hardware is modified. It also provides a simple method to correct measured performance to standard conditions.