Since the late 1960s, fungicide resistance in phytopathogenic fungi has become one of the major problems worldwide in crop protection. In the case of benzimidazoles, for example, the problem of resistance occurred shortly after introduction of the fungicides, making disease control difficult with this class of fungicides. The basic studies on biochemical genetics of fungicide resistance are indispensable for the application of biotechnological techniques to monitor the resistance in practice. Biotechnological approaches to detect fungicide resistance are only feasible where the resistance mechanism is elucidated at a molecular level. The chapter describes conventional biological methods for monitoring resistance. However, such methods are generally time-consuming, laborious, and allow relatively few strains to be monitored. Techniques based on DNA probe technology have been successful in the early diagnosis of human genetic disorders. An oligonucleotide probe based on the difference of sequence between resistant and sensitive strains will help the development of a new monitoring system.