Safe, reliable and flexible reactor operation with optimal fuel utilization is an invariable utility requirement. Current market demands such as further increase of enrichment and burnup, flexible cycle length, power uprate, MOX and reprocessed uranium fuel (ERU) assembly insertion imply demanding fuel design conditions and, as a consequence, great challenges to the fuel assembly and core design methods. In this paper the Framatome ANP’s BWR methodology COMPASS (comprehensive BWR program assembly for steady state and safety analysis) is presented together with results of recently performed validation and verification work. The paper demonstrates that COMPASS meets the challenges in all areas of steady state and transient fuel assembly and core analysis: neutronic, thermal hydraulic and mechanical fuel assembly analysis, in-core fuel management analysis and core monitoring, core transient analysis including stability, and plant transient analysis. This is achieved by employing advanced physical models and by extensive validation of the methodology. The validation is based on experimental programs, measurements at Framatome ANP test facilities, and most important, comparison of the predictions with a great wealth of measured data gathered from BWR plants during many years of operation.