Carbon nanotubes (CNT) are one of the most promising engineered nanomaterials. CNT are tubular fiber shaped engineered graphites and produced in two common types: single wall carbon nanotube (SWCNT) and multi wall carbon nanotube (MWCNT). Unfortunately, their unique properties, making CNT valuable in a large number of applications, can be problematic for their potential toxicity. Moreover, CNT share some physic-chemical properties with asbestos, a known carcinogen fiber if inhaled. In the current presentation, the genotoxicity of CNT in vitro will be discussed along with specific consideration concerning the validity of the FpG comet assay in CNT exposure. We have exposed MWCNT and SWCNT in human bronchial epithelial cell line (16HBE) and in human monocytic cell line (THP-1). FpG comet assay and micronucleus assay were performed in order to determine oxidative DNA strand breaks and aberrant micronuclei formation induced by CNTs. CNT can induce strand breaks and oxidative DNA damage. Both types of CNT induced a significant increase in % Tail DNA and Tail Moment in both cell types, whether or not in the presence of FpG. The micronuclei formation in the presence of cytochalasin B was apparent in all exposed 16HBE cultures, but not in THP-1 cultures. Without cytochalasin B, only MWCNT exposed 16HBE cells and the highest dose of SWCNT in THP-1 showed a significant increase in micronuclei. In conclusion we have observed both DNA damage and aberrant micronuclei formation in 16HBE cells. In THP-1 cells, although oxidative DNA damage and strand breaks occurred, aberrant micronuclei formation occurred in less extent. In all instances, interaction between CNT and assay system was avoided by adjusted protocols, and will be discussed during the presentation.