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Nitrogen use efficient (NUE) wheat varieties have potential to reduce input costs for growers, limit N runoff into water ways, and increase wheat adaptability to warmer environments. Previous studies have done little to explain the genetic basis for NUE and components, nitrogen uptake efficiency (NUpE) and nitrogen utilization efficiency (NUtE). Four studies were conducted to 1) determine genotypic stability of NUE under high and low N regimes and under warming 2) determine effect of warming on NUE 3) indentify QTL associated with NUE components 4) assess the utility of canopy spectral reflectance (CSR) as a high-throughput phenotyping device for NUE. Genotypic response to N stress or warming varied. Uptake efficiency was found to be more important than utilization efficiency to genotypic performance under high and low N environments and under warming. Selection under low N for NUpE and under high N for NUtE most efficiently identified NUE varieties. Uptake and utilization were lower under warming due to quickened development. No strong correlations between the CSR indices and NUE existed. No QTL were found to be significantly associated with NUE components. Further research into the mechanisms controlling NUE and to reveal plant response to N stress and under warming is necessary.