Technical Abstract: Plant resistance (R) genes provide effective protection against invading pathogens at the front line of defense. Advances in genomic technology have accelerated efforts to characterize a wide range of crop R genes from diverse and economically important crops, resulting in effective crop protection. Thus far, over 100 plant R genes have been molecularly cloned. Most cloned plant R genes were predicted to be receptor proteins with nucleotide binding sites and a leucine rich repeat domain (NBS-LRR). In contrast, the effectors that trigger plant R gene-dependent immunity are random molecules and rapidly evolve. Understanding how plant R genes evolve to recognize the effectors is a challenge in plant biology. The map-based cloning technique is the most commonly used method for plant R gene isolation. This technique involves identification of candidate R genes based on integrated genetic and physical maps using a mapping population and verification of resistant function of R genes using genetic transformation. The techniques for map-based cloning, and structure and function of R genes with emphasis on R genes to the rice blast fungus, the most devastating fungal pathogen of rice, are reviewed.