Carbon nanotubes (CNTs) are nanosized cylindrical hollow tubes consisting entirely of the element carbon. Currently, CNTs are playing an important role in drug delivery as a carrier system because of their several unique physical and chemical properties. Studies show that CNTs are toxic and that the extent of that toxicity depends on properties of the CNTs, such as their structure (single wall or multiple wall), length and aspects ratios, surface area, degree of aggregation, extent of oxidation, bound functional group(s), method of manufacturing, concentration, and dose. People could be exposed to CNTs either accidentally by coming in contact with the aerosol form of CNTs during production or by exposure as a result of biomedical use. Numerous in vitro and in vivo studies have shown that CNTs and/or associated contaminants or catalytic materials that arise during the production process may induce oxidative stress, prominent pulmonary inflammation, apoptosis in different cell types, and induction of cytotoxic effects on lungs. Studies on the toxicity of CNTs have mainly focused on the pulmonary effects of intratracheal or pharyngeally administered CNTs. This review examines the potential pulmonary toxicity of CNTs. FROM THE CLINICAL EDITOR: Carbon nanotubes are promising drug delivery agents; however, their pulmonary toxicity may represent a substantial limitation to their applicability. This detailed review discusses critical aspects of the above problem.