Light micrographs of the anatomical features of brown rot, white rot and soft rot are presented here to facilitate easy identification of each type of decay in birch and pine. This paper presents the light-microscopic observations made during the course of several broad studies of wood deterioration by fungi. A key aids the identification of brown rot, white rot and soft rot in wood and wood products. Features used for identification include bore hole size and frequency, shape of erosion channels and cavities, cell separations and changes in birefringence as observed on unstained sections with polarized light or differential interference contrast microscopy. Included are descriptions of white-rot and soft-rot erosion patterns at several decay stages.