Because of their unique physicochemical properties, engineered nanoparticles have the potential to significantly impact respiratory research and medicine by means of improving imaging capability and drug delivery, among other applications. These same properties, however, present potential safety concerns, and there is accumulating evidence to suggest that nanoparticles may exert adverse effects on pulmonary structure and function. The respiratory system is susceptible to injury resulting from inhalation of gases, aerosols, and particles, and also from systemic delivery of drugs, chemicals, and other compounds to the lungs via direct cardiac output to the pulmonary arteries. As such, it is a prime target for the possible toxic effects of engineered nanoparticles. The purpose of this article is to provide an overview of the potential usefulness of nanoparticles and nanotechnology in respiratory research and medicine and to highlight important issues and recent data pertaining to nanoparticle-related pulmonary toxicity.