The series of volumes entitled Biological Responses in Cancer provides information on approaches through which the interaction between neoplas- tic and normal cells may be modified. Topics discussed in various volumes include immunological and host defense systems, control mechanisms of cell and population growth, cell differentiation, and cell transformation. This volume is specifically concerned with various aspects of cell interactions and regulation within heterogeneous tumor cell populations, and their role in tumor progression and metastasis. Knowledge in this area is likely to provide new leads toward the exploitation of novel cellular sites and mech- anisms in the development of new types of therapies of cancer. Several topics are discussed within these general areas of consideration. The possibly unique characteristics and mechanisms of tumor vascularization and the potential sites of interference with angiogenesis that might have therapeutic impli- cations are critically evaluated. Tumor cell-normal tissue interactions in- volved in different phases of the growth and metastatic processes are dis- cussed in two chapters dealing with mechanisms of tumor invasion and with the role of collagen in mammary tumor growth; here again potential leads are identified that may be exploited toward the development of new thera- peutic approaches. The evolution of phenotypic diversity as a phenomenon complicating the biology of tumor metastasis and consequently affecting the opportunities offered by chemotherapy is also critically considered.