An important point emerging from the literature on tumor invasion in vivo is the great variability of nearly all aspects studied. It seems that there is neither one particular morphologic change which renders a cell invasive, nor one particular mechanism by which a cell crosses the boundaries of its original tissue compartment to occupy another. Nevertheless, some general trends are demonstrable. The majority of invasive tumor cells appear to be characterized by prominent surface protrusions, decreased junctional contacts and, in the case of epithelium-derived tumor cells, an incomplete basement membrane. The fact that some tumors can invade foreign tissues without loosing their basement membrane is emphasized. Invasive cells frequently form organized associations with preexistent non-neoplastic cells without damaging them. Apparently, the eventual disappearance of the preexistent cells in most invaded tissues is not necessarily due to a direct action on the part of the tumor cells. It rather seems a secondary phenomenon caused by, e.g., the insertion of invasive tumor cells between the preexistent cells and their original stroma. Very often, this seems to be due to the affinity of malignant cells for basement membranes. In addition, the adhesion of tumor cells to basement membranes frequently seems to determine their pattern of spread through a tissue. A process which may turn out to be a key factor in tumor invasion is desmoplasia, the series of host reactions which creates a new environment for the tumor cells which may favor their survival, proliferation, and locomotion. With the rapid development of new techniques, electron microscopy will probably contribute to the elucidation of the exact nature, the degree of similarity to granulation tissue, and the influence on invasion of desmoplastic tumor stroma.