Propellers and fans have in the past been tested under static conditions to provide test data for comparison with noise predicted using empirical or theoretically based procedures. This has resulted in noise prediction methodology adjusted to correlate with static test data. Recent tests have shown that the noise of propulsors in flight differs significantly from propulsors tested statically. This paper discusses the current knowledge of propulsor noise from an experimental and analytical standpoint in both static and forward flight operating regimes. Also, the advantages and disadvantages of various approaches to conducting definitive forward flight tests are presented. Emphasis is placed on evaluation of propellers and fans operating at subsonic tip speeds where, experimental and theoretical developments can most easily be applied.