Carbon nanotubes were discovered in 1991. Since then researchers have been exploring their potential for applications in the area of biological and materials sciences. Research carried out by chemical modification and bio-functionalization of these materials has made it possible to generate a new class of bioactive carbon-nanotubes. These can easily be linked to proteins, carbohydrates or nucleic acids by developing nanoscale bioelectronics systems and biosensors for the targeted bio-delivery applications to convey peptides or other genetic materials to cells. Using carbon nanotubes and adopting advanced nanoscale fabrication methods biosensors are designed even to detect single-molecule. Carbon-nanotube and DNA can be used to make hybrids. Nanotube-based optical sensors capable of exquisite detection sensitivity at the molecular level have also been developed.