As a discipline, cell biology has classically been defined by the boundaries which define the limits of its interests. These boundaries may apply to location -cell biologists study material within the cell - or they may apply to size - cell biologists study material above the level of the macromolecule. For many years, the limits of technique enforced these boundaries as surely as any custom of the discipline. Our inability to investigate structures which could not be observed or molecules which could not be detected assured that cell biology would not cross the boundaries which linked the cell to other levels of organization. As the twentieth century nears its close, however, the development of an enormous range of tools and techniques, some physical, some chemical, some biological, has changed this situation forever. Cell biology today crosses the boundary, links the molecule with the organelle, associates the cellular response with the larger organism. For some, these advances have produced a sense of consternation as it becomes increasingly difficult to define the exact distinction between a cell biologist, a biophysicist, and a molecular biologist. But for many others, the same crisis of identity is perceived as a great opportunity, an opportunity to make connections at a range of levels which integrate our knowledge of living systems in a way that the pioneers of our field could hardly have imagined. I hold to this latter view, and I suspect that nearly all of the contributors to this volume do as well. Each of the articles within this volume cross boundaries in their efforts to deal with the complexities of the living cell. As examples, the work of Carl Cohen illustrates the emergence of a new understanding of the interface between cell membrane and cytoskeleton. Robert Murphy's studies follow the movement of material into the cell and through a maze of cellular compartments, and the studies of Vivianne Nachmias and Ken-ichi Yoshida illustrate the influences exerted by a range of proteins on shape changes in platelets. Each of the studies in this volume has, in a certain sense, crossed the traditional limits of study and made an important contribution to developing a complete and dynamic picture of biology at the level of the cell. In its own way, each article points towards the ultimate goal of cell biology: an understanding in which all of the boundaries which separate organism from cell and cell from molecule will have been breached. That goal is not yet in sight, but the journey is only beginning.