Evaluation of neutron fluence for reactor vessels is important for evaluation of irradiation damage that may limit vessel life. The uncertainty in calculated fluence is considered to be typically about 20%. Benchmark measurements have improved confidence in the uncertainty assessments and enable testing of improved cross section data and calculational procedures. Use of appropriate plant-specific measurements can lower uncertainty substantially below 20% for some cases and also can limit the effects of unknown errors that may cause larger deviations.