Although many plant pathogens and plant pests are known to develop resistance to plant protection products such as pesticides, their resistance to biocontrol agents is still relatively unexplored. However, cases of resistance (or reduced sensitivity) to biocontrol agents have been observed for insect pests under field crop conditions, with products such as microorganisms, plant or microbial extracts, semiochemicals and beneficial insect parasitoids. The lack of documented cases of resistance of weeds and pathogens (bacteria, fungi, oomycetes, nematodes, viruses) to biocontrol products may be explained by their still limited use in agriculture compared to the use of biocontrol against insects. However, some studies show differences in the susceptibility of various isolates of plant pathogens to biocontrol agents or to compounds that are produced or synthesized as a result of their interaction with the host plant. This chapter highlights current knowledge regarding the erosion of biocontrol of plant pests and its possible consequences on field efficacy.