This paper attempts to present a clearer picture, a detailed and up to date review of the heat transfer enhancement and thermal conductivity improvement for conventional fluids by adding carbon nanotubes or hybrid carbon nanotubes in the base fluid to obtain nanofluids or ionanofluid. Carbon nanotubes have attracted the interest of different researchers because of their high thermal conductivity that exceeds other equivalent types of nanoparticles. In view of this, the effect of different key factors like concentration, temperature and shape type of nanoparticles on the thermal conductivity improvement in nanofluids were reviewed. Moreover, the effect of surfactant stabilizers on the carbon nanotubes nanofluids distribution was evaluated. The results that have been obtained from the valuable studies have been analyzed and some gaps have been found that need to be re-reviewed by the researchers.