Although some widely used paradigms have been identified, plant disease resistance operates by a multitude of mechanisms. We have been studying the unique mechanisms of resistance to the most damaging disease of soybean, caused by soybean cyst nematode. We found that the most useful resistance to SCN, which is present in many of the most successful soybean varieties, relies on an intriguing form of copy number variation involving up to ten copies of a 31kb genome segment that encodes four disparate proteins. Our most recent work on this Rhg1-mediated resistance will be discussed. In a separate project we have found that diverse plant pathogens, early in the infection process, induce DNA double-strand breaks in the host plant genome. Plants require the capacity to limit or repair this DNA damage as part of an effective immune system, and our more recent work on these processes will be presented in the second part of the seminar.