The literature on P.pini is reviewed. The mycelium and spore stages of P.pini are described. Extension of the lesion takes place by growth of the hyphae along the edge of the functional phloem. Both longitudinal and centripetal growth ocours in the xylem along the resin canals. Tissues formed by a diseased cambium are severely altered. In xylem tracheids decrease in length and diameter, rays increase in height and number, ray cells increase in width but not in height, fusiform rays increase in number and diameter. In phloem formed by discased cambiun, longitudinal parenchyma cells are larger and more numerous, while changes in phloem rays are similar to those of the xylem rays. The bi- or tri-nucleate aeciospores produce an aseptate bi- or tri-nucleate germ tube. On vesicle formation, the nuclei in the germ tube split into 8 individual chromosomes which fuse to form 4 tetrads. These move out of the vesicle into the tube beyond, divide twice, and give rise to 4 nuclei. The process is parallel to meiosis except that no diploid interphase nucleus is formed. 10-15 percent of natural lesions result from wound infection. The remaining are associated with unwounded needle bearing shoots. The differences in symptoms between these two types of infection, the formation of vesicles in stomatal cavities of needles, and the presence of a single needle with mycelium in its vascular tissue on each lesion, indicates that the latter type of lesion results from needle infection. The development of lesions on trees, particularly the factors which determine the probability that a given branch lesion will reach the main stem, are discussed, using the data from 270 natural lesions in two pine stands.