The carbon nanotubes (CNTs) have emerged as novel functional materials with the progress of materials science and nanotechnology. The CNTs are sp 2 hybridized rolled up graphene sheets having one-dimensional carbon possessing aspect ratio > 1000. The important class of CNTs like single walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) etc ., has been briefly outlined. This chapter covers the most significant as well as efficient chemical or physical functionalization approaches for obtaining desirable properties of mostly SWNTs and MWNTs based on covalent and non-covalent interactions. The desired properties such as chemical, physical, mechanical, electrical, transportation properties, etc ., of the CNTs can be achieved through its controlled functionalization. The mechanism of the functionalization of CNTs has been discussed briefly. The advantages and disadvantages of various chemical functionalization methods of CNTs have also been overviewed. The outlook of its future applications based on its functionalization has been predicted.