A series of pathological changes occur in pine trees affected by the pine wood nematode (PWN, Bursaphelenchus xylophilus) through the interaction between the nematode and pines, that is, the action of the PWN and host responses. Included in such changes are: the action of the pathogen, host responses which induce and accelerate disease development leading to host death, by-products of the wilting process, and host defense responses. Substances toxic to pines and defense-related substances have also been reported, and the role of such substances has been investigated in different pine species and in various environments. Responses, especially biochemical ones, occurring in pine trees during the development of pine wilt disease and during the development of resistance are described here.