Detailed performance analysis of centrifugal compressors in the field essential to evaluate their existing condition. The current performance of a compressor can also be a valuable tool in evaluating its reliability. A decrease in compressor performance can be an excellent indication of internal wear of fouling, which if allowed to continue may result in unscheduled outages or reduced throughput. In contrast, perceived performances problems may be a result of a compressor operating far from its original design. Obtaining accurate performance data in the field can be very challenging. The author explains the relative importance that different process variables have in the performance calculations, as well as specify the necessary instrumentation to obtain process data with an acceptable uncertainty. Normal ranges and limitations for calculated head and efficiency are provided to assist users in determining if the field data are realistic. Methods to estimate both mechanical and seal losses are demonstrated. Since original design condition almost never match actual operating conditions, the author demonstrates how to compare actual field data with design data, using nondimensional head and efficiency. Likewise, the limits on these comparisons are outlined for users. The author provides several example field performance evaluations and discusses ways to avoid some common pitfalls. Examples of the effects of inaccurate process data are also included in the discussion.