The primary mode of action of all benzimidazoles has been identified as the specific binding to the β-tubulin subunit of fungal tubulin and, consequently, an interference with microtubule assembly. The genetics of fungicide resistance are often discussed in connection with the development of resistance in the field. Genetics are also regarded as an important factor, when the risk of fungicide resistance is assessed. To cope with the problem of fungicide resistance, strategies such as alternating or mixing unrelated fungicides are usually recommended. However, these strategies would only delay and not avoid the emergence of resistance. Benzimidazoles exert their antifungal activities by binding to β-tubulin. The chapter presents the molecular mechanisms of resistance proposed for plant pathogenic fungi. The cloning of β-tubulin genes has been accomplished for several plant pathogenic fungi such as Erysiphe graminis forma specialis.hordei,Septoria nodorum, and Venturia inaequalis.