Carbon nanotubes (CNTs) are a man-made form of carbon, they exist in many different forms and can be chemically modified and/or functionalized with biomolecules. Pristine single-walled CNTs (SWNT) are extremely hydrophobic tubes of hexagonic carbon (graphene) with diameters as small as 0.4 nm and lengths up to micrometers. Multiwalled CNTs (MWCNT) consist of several concentric grapheme tubes and diameters of up to 100 nm. Due to their unique chemical, physical, optical, and magnetic properties, carbon nanotubes have found many uses in industrial products and in the field of nanotechnology, including in nanomedicine. From the rapidly emerging applications of CNTs arise the urgent need for toxicological studies on CNTs. Exposure to CNTs is associated with effect on DNA, pulmonary toxicity, fibrotic lesions, skin toxicity. Very interesting data are collected in a study in which CNTs were seen to connect multiple cells together. Of greatest interest was the discovery of unique intercellular carbon structures composed of SWCNT that bridged lung macrophages. These "carbon bridges" offer a novel and easily identifiable biomarker of exposure. This review brings together toxicological studies that have been carried out using carbon nanotubes, to offer a summary of the state-of-the-art in the development of carbon nanotubes toxicology.