The potential environmental and health impacts of nanotechnologies triggered a recent surge of life cycle assessment (LCA) studies on nanotechnologies. Focusing on the energy use and greenhouse gas emissions impacts, we reviewed 22 LCA-based studies on nanomaterials, coatings, photovoltaic devices, and fabrication technologies that were published until 2011. The reviewed LCA studies indicate that nanomaterials have higher cradle-to-gate energy demand per functional unit, and thus higher global warming impact, than their conventional counterparts. Depending on the synthesis method, carbon-based nanoparticles (i.e., carbon nanofibers, carbon nanotubes, and fullerenes) require 1 to 900 gigajoules per kilogram (GJ/kg) of primary energy to produce, compared with ∼200 megajoules per kilogram (MJ/kg) for aluminum. This is mainly attributed to the fact that nanomaterials involve an energy-intensive synthesis process or an additional mechanical process to reduce particle size. Most reviewed studies ascertain, however, that the cradle-to-grave energy demand and global warming impact from nanotechnologies at a device level are lower than from conventional technologies because nanomaterials are typically used in a small amount to improve functionality and the upgraded functionality offers more energy-efficient operation of the device. Because of the immature status of most nanotechnologies, the studies reviewed here often rely on inventory data estimated from literature values and parametric analyses based on laboratory or prototype production, warranting future analyses to confirm the current findings.