A better understanding of the genetic basis underlying the genotype dependence of Brassica oleracea transformation is enabling researchers to distinguish between recalcitrant and successful candidate genotypes for routine transformation. In this chapter we outline an A. tumefaciens-mediated transformation method for B. oleracea using 4-d-old cotyledonary explants and a model B. oleracea doubled haploid genotype, AG DH1012. After only 3 wk on kanamycin selection, the first transgenic shoots can be isolated. Transformation efficiencies in the region of 10 to 25% (based on 50-125 PCR-positive independent shoots from 500 infected explants) are typically achieved. For researchers wishing to use their own plant genotype, we highlight the tissue culture phenotypes that are conducive to efficient transformation.