In the last decade, our view of intermediate filaments (IFs) has changed dramatically. Once thought of a static structures, with ill-defined functions, it is now clear that IFs are quite dynamic and undergo dramatic reorganizations in response to cell cycle signals, cellular differentiation, and pathogenic events. Moreover, IFs have been shown to play important architectural roles in epidermis, neurons and muscle. At the same time, the roles of IFs in a number of other cell types, e.g. glia, fibroblasts, simple epithelia, remains much more enigmatic. The more general questions of how IF protein expression and network formation influence a cell's oncogenic potential and behavior (the subject of this volume) remains largely unclear. As a general introduction to this volume I will briefly describe the known IF proteins and their intracellular organization.