This chapter explores both plant pathologists and breeders as to the potential use of biochemical markers in breeding programs. Marker identification, as evidence of the presence or an indication of the character, is the basic approach adopted to bring together a broad spectrum of possibilities to detect resistance by means of "biochemical" assays rather than by the conventional "biological" host-pathogen interactions, as expressed by artificial or natural inoculations. Peroxidase (POX) activity is frequently increased in plants infected by pathogens, and the level of its activity is often closely correlated with disease resistance, as documented by T. Kosuge more than 20 years ago. In lettuce, a trend was apparent indicating that one component of field resistance to Bremia lactucae could be related to a high level of POX prior to infection. Chitinase, a hydrolase with an antifungal potential, is one of the pathogenesis-related proteins which is induced in cucumber seedlings94 or tobacco plants in response to infections producing necrotic symptoms.