Intermediate filaments (IFs) are the most enigmatic component of the eukaryotic cytoskeleton. IFs are often a major component of the cell and are most prominent in the cells of vertebrates. Ultrastructurally similar cytoplasmic filaments have been found in many metazoan phylla and some protists and homologous proteins form the nuclear lamina, a fibrous structure present within the nuclei of most, if not all, eukaryotic cells [see Franke, 19871. However, defining the functions of IFs has been quite difficult due in large measure to the absence of membrane-permeable drugs that spec$cally disrupt IF organization. At present, the only means of disrupting IF organization specifically appears to be through either the analysis of genetic mutations in IF subunit proteins or the intracellular injection of anti-IF antibodies (see below). Whereas nuclear IFs (nIFs) appear to play a key role in nuclear organization [see Benavente and Krohne, 19861, the absence of significant changes in cellular behavior following the disruption of cytoplasmic IF (cIF) organization in cultured cells, together with the identification of a number of mammalian cell lines [see Paulin-Levassear et al., 1988, and references therein] and normal cell types [see Bartnik et al., 1986, 19871 that lack cIFs, indicates that cIFs are not critical to the viability of eukaryotic cells. Nevertheless, homologous IF gene structure, protein sequence, and patterns of cell-type expression are highly conserved [see particularly Hermann et al., 1989a,b], a fact that implies that IF proteins perform important functions, if not within the isolated cell then at the level of tissues, organs, and the organism. In the first part of this review, we discuss what an IF is and the basic patterns of IF assembly, dynamics, and organization. In the second part, we examine how these aspects of IF cell biology have informed our thinking on IF functions. Finally, we summarize the methods currently available for the experimental study of IF function. We will not discuss the complex question of IF gene