The advent of high-throughput genomic technologies and the availability of a reference genome sequence of Brassica napus and its diploid parental species, B. rapa and B. oleracea, open new insights into the genomic localization of agronomic trait-associated quantitative trait loci (QTL), the identification of underlying genes and their sequence variation. Over the last 20 years, many genetic maps of B. napus have been built, progressively integrating various types of markers. Large single-nucleotide polymorphism (SNP) arrays allowed the construction and integration of high density maps and their anchorage to the B. napus sequence. Increasingly, precise genetic analyses of agronomic traits could thus be carried out, either through linkage analysis or through genome-wide association mapping. Comparative genomics allowed the genomic localization of the genes and QTL controlling agronomic traits, as well as an assessment of the impact that the high level of duplications present in this polyploid species has on the genetic architecture of the traits and on the structural and functional diversity of the genes involved. This chapter reviews the evolution of B. napus genetic and genomic resources and their use in gene and QTL mapping for several major traits and then shows how the availability of the B. napus genome sequence allows more accurate investigation of the genomic regions and underlying genes involved.