High aspect ratio, or fiber-shaped, nanoparticles (HARNs) represent a growth area in nanotechnology as their useful properties become more apparent. Carbon nanotubes, the best known and studied of the HARNs are handled on an increasingly large scale, with subsequent potential for human inhalation exposure. Their resemblance to asbestos fibers precipitated fears that they might show the same type of pathology as that caused by asbestos and there is emerging evidence to support this possibility. The large number of other HARNs, including nanorods, nanowires and other nanofibers, require similar toxicological scrutiny. In this article we describe the unusual hazard associated with fibers, with special reference to asbestos, and address the features of fibers that dictate their pathogenicity as developed in the fiber pathogenicity paradigm. This paradigm is a robust structure:toxicity model that identifies thin, long, biopersistent fibers as the effective dose for fiber-type pathogenic effects. It is likely that HARNs will in general conform to the paradigm and such an understanding of the features that make fibers pathogenic should enable us to design safer HARNs.