Described are procedures to be followed in applying cross-section sensitivity and uncertainty analysis to arrive at estimates for the uncertainty associated with an integrated reactor performance parameter R. Means to reduce such uncertainty are outlined. The pertaining computational codes used as analytical tools to carry out the cross-section sensitivity/uncertainty analysis are reviewed with emphasis on their current limitations and possible improvements. The range of the uncertainty in the prediction of the tritium breeding ratio TBR due exclusively to current uncertainties in the nuclear data field is presented as an application for the sensitivity and uncertainty methods and codes.