Various applications of carbon nanotubes require their chemical modification in order to tune/control their physicochemical properties. One way for achieving this control is by carrying out doping processes through which atoms and molecules interact (covalently or noncovalently) with the nanotube surfaces. The aim of this chapter is to emphasize the importance of different types of doping in carbon nanotubes (single-, double- and multiwall). There are three main categories of doping: exohedral, endohedral and inplane doping. We will review the most efficient ways to dope carbon nanotubes and discuss the effects on the electronic, vibrational, chemical, magnetic and mechanical properties. In addition, we will discuss the different ways of characterizing these doped nanotubes using spectroscopic techniques, such as resonant Raman, X-ray photoelectron, electron energy loss spectroscopy and others. It will be demonstrated that doped carbon nanotubes could be used in the fabrication of nanodevices (e.g., sensors, protein immobilizers, field emission sources, efficient composite fillers, etc.). We will also present results related to the importance of inplane-doped nanotubes for attaching various metal clusters and polymers covalently using wet chemical routes.