Exposure to fast neutron fluence causes damage to reactor vessels, and this damage must be constrained within limits for safe operation. Therefore, the fluence becomes an important quantity when such limits are approached. This paper discusses some impacts of fluence uncertainty on where these limits are set. Improvements in fluence assessment in recent years may indicate that fluence margins can be reduced, but detailed probabilistic considerations that take into account how well uncertainties are known may play an important part in quantifying safety margins.