In this part, we have reviewed nematodes and microorganisms, such as bacteria and fungi, related to pine wilt disease, but we have not reported on entomopathogenic fungi, bacteria, viruses, or protozoa associated with Monochamus beetles as these are reviewed in Chap. 35. In Chap. 27, Zhao reviews the possibility that mutualistic symbiosis exists between the PWN and its associated bacteria, showing that they enhance one another’s reproduction. In contrast, some PWN-associated bacteria have no beneficial effects but rather they are detrimental to the PWN. Zhao suggests that the mutualistic symbiosis between the PWN and some bacteria might result from coevolution over a long time, because it is difficult to imagine that PWN and accidentally contaminating bacteria could establish such a mutualistic association. As Zhao points out, intensive research on the interactions between the PWN and bacteria has just started and further research is needed.