Black leg of canola (Brassica napus), caused by Leptosphaeria maculans, is primarily controlled by the use of resistant cultivars. In Canada, resistance has been effective for about 10 years, but the frequency of new pathotypes is increasing and there are reports of breakdown of resistance in some cultivars. Race-nonspecific resistance is present in many cultivars in Australia and Europe. The genetic basis of resistance in Canadian cultivars is unknown. This paper reviews our current knowledge relating to disease management using resistance in Canada and the genetics of race-specific and race-nonspecific resistance in the host-pathogen interaction between L. maculans and B. napus. The particular focus is on resistance associated with the A genome and how resistance might be achieved over the long term.