The ‘Precautionary Principle’ provides a somewhat ill-defined guide, often of uncertain normative status, for those exercising administrative decision-making power in circumstances where that may create potential risks to human health or the environment. This paper seeks to explore to what extent the precautionary principle should have been and was in fact utilised by the Australian Therapeutic Goods Administration (TGA) in its decision to approve the marketing of sunscreens containing titanium dioxide (TiO2) and zinc oxide (ZnO) in nanoparticulate form. In particular, (...) this article assesses to what extent better application of that principle might have altered the TGA’s decision that TiO2 and ZnO ENPs in sunscreens do not require new safety testing, because they are considered to be functionally equivalent to their bulk counterparts. (shrink)