Over the last decade the development and use of nanomaterials (NMs) and nanoparticles (NPs) has increased at a great rate. As a result there is an ever increasing risk of exposing humans and wildlife to these potentially harmful materials. Titanium dioxide nanoparticles (TiO2 NPs) and carbon nanotubes (CNTs) are two of the most widely used NMs at present. Their potentially harmful effects on organisms and physicochemical properties have been investigated in a growing number of scientific studies. However understanding the level of risk they may pose is far from satisfactory. The present body of work has addressed various aspects of this field. In order to better quantify the fate of TiO2 NPs in the environment the methodology of measuring Ti from TiO2 NPs was improved using ICP-OES and single particle ICP-MS was demonstrated to provide the first steps towards characterising the nature of TiO2 NPs in liquid-phase media. The potential harm of TiO2 NPs and single walled carbon nanotubes (SWCNTs) to zebrafish was investigated in two separate studies. Little evidence of physiological toxicity was found and the only nano-scale effect of note was an increase in total glutathione of zebrafish exposed to TiO2 NPs. More subtle effects in reproductive studies were further investigated using the three-spined stickleback in a longer term investigation. Similarly to the zebrafish there was little evidence of any physiological disturbances and the well documented reproductive behaviour of the stickleback was not significantly altered as a result of TiO2 NP exposure. This body of work has added to the understanding of the potential toxic effects caused by exposure to both TiO2 NPs and SWCNTs. Improved methods for the detection and characterisation of TiO2 NPs have been demonstrated and the most sensitive tools for ecotoxicological assessments of NP toxicity have been elucidated.