Understanding the molecular basis of disease-resistance genes has long been a goal of plant pathology. With the advent of gene cloning technology in the 1970s, this goal finally became realistic and in the intervening years several important host defense-related genes have been cloned and characterized. Plant disease-resistance genes have been identified based on their genetic phenotype. However, because so little was known about their biochemical function, it has been impossible to clone these genes using conventional gene cloning techniques. By contrast, the two gene cloning strategies described in this chapter, map-based cloning and transposon tagging. Nevertheless, map-based cloning and transposon tagging, along with techniques derived from them, will certainly form the foundation for resistance gene cloning in plants in the years to come. Map-based cloning is based on the ability to identify DNA genetic markers that are very tightly linked to the gene of interest.