For the removal and separation of wastewater pollutants, one widely used technique is adsorption. Different materials have been used as adsorbents. Among several adsorbents, carbon nanotubes (CNTs) are emerging as potential adsorbents because of their large surface area, defined cylindrical hollow structure, high ratio aspect, easy modification and hydrophobic wall surfaces. In this review, the capability of adsorption of dyes on CNTs and functionalized CNTs has been compiled. This paper provides data about defects, adsorption sites on CNTs and adsorption process studies under the effect of such various operational parameters as solution pH, contact time, and temperature, and deals with the mechanisms involved in the adsorption of dyes onto CNTs. According to observations from the articles reviewed, functionalized carbon nanotubes have better sorption capacity than as-grown CNTs. For the removal of dyes from aqueous solution, carbon nanotubes (CNTs) have shown good potential and some of the functional groups improve the adsorption potential according to evidence from the literature. For example, the maximum adsorption amount (q_max) of methylene blue dye on an MWCNT surface has been reported to be 100 mg g⁻¹ while the maximum adsorption amount of methylene blue dye on functionalized MWCNT–SH surface was 166.7 mg g⁻¹. This indicates that functionalization is beneficial in increasing the amounts of absorption. However, the development of highly efficient, cost effective environmentally friendly CNT-based nano adsorbents for their commercial applications should be the focus of still more study.