There has been a considerable scientific, governmental, and public interest in potential adverse health effects associated with exposure to engineered or synthesized nanomaterials. Although such effects have not been reported in humans, there is accumulating evidence from animal studies that exposure to some nanomaterials may be harmful. There is sparse knowledge as to the likelihood, frequency, and intensity of exposures experienced by those working around engineered nanoparticles. Similarly, there is little knowledge regarding the potential existence, type, and dose dependence of adverse health effects, which might result from workplace exposures to engineered nanoparticles. This uncertainty, reflecting a relative lack of research, makes it difficult at present (and probably for the near future) to fully rely upon firm scientific evidence for the development of rational, preventive, and screening measures to protect against such potential effects. Recognizing this predicament, the American College of Occupational and Environmental Medicine (ACOEM) has developed this guidance document for occupational medicine physicians and their colleagues. The purpose of the document is to offer current prudent preventive recommendations on the topics of exposure monitoring, exposure controls, and medical surveillance. This document will not attempt to review the rapidly evolving animal toxicology literature in detail, as any review would be quickly outdated, but general areas of concern will be discussed.