Cell morphology, architecture and dynamics primarily rely on intracellular cytoskeletal networks, which in metazoans are mainly composed of actin microfilaments, microtubules and intermediate filaments (IFs). The diameter size of 10 nm - intermediate between the diameters of actin microfilaments and microtubules - initially gave IFs their name. However, the structure, dynamics, mechanical properties and functions of IFs are not intermediate but set them apart from actin and microtubules. Because of their nucleotide-independent assembly, the lack of intrinsic polarity, their relative stability and their complex composition, IFs had long been overlooked by cell biologists. Now, the numerous human diseases identified to be associated with IF gene mutations and the accumulating evidence of IF functions in cell and tissue integrity explain the growing attention that is being given to the structural characteristics, dynamics and functions of these filaments. In this Primer, we highlight the growing evidence that has revealed a role for IFs as a key element of the cytoskeleton, providing versatile, tunable, cell-type-specific filamentous networks with unique cytoplasmic and nuclear functions.