We review the mechanisms and pathways whereby nanoparticles might cause genotoxicity. Primary and secondary mechanisms are discussed in relation to the general particle toxicology paradigm. We also discuss how we might improve genotoxicity assays for nanoparticles. In this context we describe the role of the dispersion and the protein corona, the most relevant metric, choice of controls and new endpoints for genotoxicity along with the need for a structure activity model of NP genotoxicity.