Carbon nanotubes (CNTs) are considered one of the most promising materials in nanotechnology, with attractive properties for many technologic applications. The different synthesis, purification, and postprocessing methods produce CNTs with different physical characteristics, which can be applied in different fields ranging from composite materials, medical applications, and electronics to energy storage. The widespread projected use of CNTs makes it important to understand their potential harmful effects. In this environmental health review we observed a remarkable range of results of some of the toxicology studies. The comparability should be improved by further standardization and introduction of reference materials. However, at present the findings of this review suggest several key points: a) there are different types of CNTs, and therefore they cannot be considered a uniform group of substances; and b) in environmental compartments, CNTs can be bioavailable to organisms. The properties of CNTs suggest a possible accumulation along the food chain and high persistence. In organisms the absorption, distribution, metabolism, excretion, and toxicity of CNTs depend on the inherent physical and chemical characteristics such as CNT functionalization, coating, length, and agglomeration state that are influenced by the external environmental conditions during CNT production, use, and disposal stages. Characterized exposure scenarios could therefore be useful when conducting toxicologic studies. However, CNTs produce a toxic response upon reaching the lungs in sufficient quantity; this reaction is produced in a time-and dose-dependent manner. The identification of possible risks to human health and environment is a prerequisite for a successful introduction of CNTs in future applications.