Nanotechnology is an emerging technology with the potential to contribute towards sustainability. However, there are growing concerns about the potential environmental and human health impacts of nanomaterials. Clearly, nanomaterials have advantages and disadvantages, and a balanced view is needed to assess the overall benefit. The current “green and clean” claims of proponents of nanomaterials across different sectors of the economy are evaluated in this review study. Focusing on carbon emissions and energy use, we have reviewed 18 life cycle assessment studies on nanomaterials in the solar, energy, polymer, medical and food sectors. We find that the “green and clean” claims are not supported for the majority of the reviewed studies in the energy sector. In the solar sector, only specific technologies tend to support the “green and clean” claims. In the polymer sector, only some applications support the “green and clean” claims. The main findings show that nanomaterials have high cradle-to-gate energy demand that result in high carbon emissions. Synthesis of nanomaterials is the main contributor of carbon emissions in the majority of the studies. Future improvements in reducing parameter uncertainties and in the energy efficiency of the synthesis processes of nanomaterials might improve the environmental performance of nanotechnologies.