The functionalization of the CNT, which involves the chemical/physical modification of the uncontaminated CNT with different types of functional groups, improves the capabilities of the CNT for desalination and / or removal of water pollutants. This document aims to provide an exhaustive review of the functional materials of the CNT (f-CNT) and its existing and potential applications in water treatment and membrane desalination processes, with particular attention to the critical evaluation of progress, and directions of future research. The CNT nanocomposite membranes have been studied on a bench scale to efficiently remove a variety of contaminants and salts from the water, while future improvements have been made with the development of CNT functionalization techniques. CNT-based membrane applications have been found to have a number of advantages, including improved water permeability, high selectivity and anti-fouling capacity. However, its large-scale applications are still limited by its high cost. Finally, we stress that f-CNT membranes with promising removal efficiencies for the respective contaminants should be considered for commercialization and to achieve a holistic performance for water treatment and desalination.