As for fullerenes, the potential and the growing use of CNT and their mass production have raised several questions about their safety and environmental impact. Research on the toxicity of carbon nanotubes has just begun and the data are still fragmentary and subject to criticisms. Preliminary results highlight the difficulties in evaluating the toxicity of this new and heterogeneous carbon nanoparticle family. A number of parameters including structure, size distribution and surface area, surface chemistry and surface charge, and agglomeration state as well as purity of the samples, have considerable impact on the reactivity of carbon nanotubes. However, available data clearly show that, under some conditions, nanotubes can cross the membrane barriers and suggests that if raw materials reach the organs they can induce harmful effects as inflammatory and fibrotic reactions. Therefore, many further studies on well-characterized materials are necessary to determine the safety of carbon nanotubes as well as their environmental impact.