The application range of carbon nanotubes (CNTs) has been significantly expanded by the advent of reliable chemical functionalization methods. This article surveys electrochemistry-based approaches that have been devised for the covalent and non-covalent modification of CNTs, and highlights their increasing importance in the development of nanoscale and macroscopic CNT devices. The primary focus is on electrochemical protocols for selective functionalization of CNTs according to their electronic properties, as well as the fabrication of various types of CNT-based sensors for gases and (bio) molecules.