Within the last 10 years, intermediate filaments (IF) has become a topic of considerable interest to cell and molecular biologists. It now appears that IF and IF-like proteins are prominent components of the cytoskeleton and nu clear envelope (karyoskeleton) of most eukaryotic cell types (1-9). A large body of data indicates that IF proteins constitute an extremely heterogeneous multigene family, of for example, 30 or more protein chains per mammalian species. In this regard, they are far more complex than other principal cytoskeletal structural proteins, which form microtubules (tubulin) or micro filaments (actin). While we do not yet fully understand the role of IF in organisms, this complexity means that IF are a functionally diverse com ponent of cells. Yet IF proteins are all built according to a common tripartite plan. They possess a central a-helical'rod'domain of conserved secondary structure that forms the basis of their related structures and morphologies. They possess amino-terminal and carboxyl-terminal'end'domains of widely varying size and chemical character. It is now widely suspected that the properties of these end domains in large part define the functional diversity of the IF family. Biochemists and biophysicists are interested in solving the structure of this complex and unique class of fibrous proteins. Molecular biologists are interested in how the various genes of IF proteins are regulated during development and differentiation. Cell biologists are interested in the relationship between differential IF gene expression in specialized cells and the dynamic organization and functions of IF in cells. Our current understanding of IF follows an hierarchical progression of complexity beginning with the structure of the IF protein chain; the formation of a-helical coiled-coils; the assembly of these into IF; the regulation of their gene expression; the distribution of IF in cells; and their interactions with other IF-associated proteins (IF AP) as well as the cell periphery and the nucleus. By and large, our understanding of IF declines progressively with these increases in organizational complexity. In this review, we summarize key data in the order of this progression, by presenting known facts and emphasizing areas of controversy and uncertainty, with the hope that this will stimulate a wider research effort into the understanding of the function of IF.