Intermediate filaments (IFs) are often viewed as forming a static cytoskeletal network that allows cells to resist mechanical stress and deformation. But they are in fact remarkably dynamic. In a Commentary on p. 133, Robert Goldman and co-workers review recent work that has revealed the complex motile behaviour of IFs in cells, which is coupled to their assembly and disassembly. Experiments using vimentin-GFP fusion proteins, for example, have shown that IFs exist in at least three different structural forms: particles, short `squiggles' and long IFs. All three forms of vimentin can associate with cytoskeletal motors such as kinesin and cytoplasmic dynein, which transport them bidirectionally along microtubules at speeds of up to 2 μm/s. Other studies have shown that keratin IFs, by contrast, probably move along microfilaments, employing actin-based myosin motors. In neurons, defective IF transport could be responsible for the accumulations of neural IFs that characterize neurodegenerative diseases such as Parkinson's and amytrophic lateral sclerosis (ALS). Indeed, mutations in the neuronal IF subunit NF-L that disrupt its axonal transport are associated with these diseases - as are mutations in the motors that transport it.