Although the field of contemporary immunobiology continues to diversify and encompass an increasing array of biomedical disciplines and topics, there are frequently several themes that will receive special emphasis and prominence at any given time. It is our hope that this series will reflect these themes and pro­ vide an appropriate venue for exposure of such topics at a useful time. Although this particular volume is not designated as one of the special topics volumes in this series, the selected topics have in essence come together to con­ sider aspects of two major areas of considerable research interest in immuno­ biology today. These concern new approaches and insights into an understanding of the tumor-host relationship, and aspects of cellular interactions and networks as approached by various different lines of investigation. The province of tumor immunology remains one of the most challenging areas to immunologists, as it of necessity involves not only developing an under­ standing of the neoplastic process itself and how the immune system responds, but of eventually using this information in a diagnostic or therapeutic manner.