Carbon nanotubes (CNTs) belong to the fullerene family, also known as graphene. These graphenes are similar to the graphite sheets and when these are turn up in the cylindrical form they are known as carbon nanotubes. Currently, the most common methods used for CNTs preparation are: Electric-arc-discharge methods, Chemical-vapor-deposition method and Laserablation method. In order to cross the cell membrane, functionalization of the pristine CNTs is performed. Because of the sp2 hybridization and closely packed hexagons in their structure, functionalization of the pristine CNTs can be done easily with either therapeutic agent or the imaging agent. They have wide applications in the field of bio-imaging because of their intrinsic optical, mechanical and electrical properties. They can be used as efficient contrast agents and the biosensors as well as efficient carriers for the delivery of therapeutic or imaging agents.