This chapter concentrates on sou-borne plant pathogens as the nontarget organisms. A great number of factors influence the response of nontarget organisms in soil or on media. Probably the most relevant factors are dose rates of the products, additives to the pesticides, and growth substrate factors like nutrient status or presence of soil colloids. The role of additives is well illustrated by the effects of antidotes to herbicides, protectants that are applied together with the pesticides in order to reduce injury to nontarget hosts. The chapter includes direct effects on susceptibility of the host plant and effects on environmental conditions in the crop influencing the infection. Pesticide effects on parasitic interactions seem to be as specific as the mycoparasitic interactions themselves. A shift in the microbial population is due to differential pesticide sensitivity either of the pathogens themselves or of the pathogens and their antagonists.