Positive displacement motors (PDM) are being used extensively due to the increased drilling of horizontal/extended reach wells as well as the deepening of vertical wells. Knowing motor performance is essential to ensure drilling success. To enhance the technology and make the system effective, a complete performance envelope of the motor in use is required. In the traditional way, the performance of a PDM can be described by pressure drop vs. torque and rotational speed. For a particular type of motor, the RPM curves are different at different flow rates. :Although it is very difficult to fully describe the motor performance with limited testing, a good prediction is critical for motor selection and operation. In addition, the performance of a PDM operating with compressible fluid decreases drastically as compared to the operation with incompressible fluids. In this paper, analysis of the test results for various PDMs from different PDM suppliers are presented. Correlations have been developed, based on the test data, to describe PDM performance with incompressible fluids.