Although most research has focused on the use of synthetic NTs as water channels, increasing attention has been given to the design of ion-selective synthetic NT channels that could potentially be useful in unique devices such as ultrasensitive biosensors, and may help to gain insight into complex biological ion channel processes. As a result of the relatively recent interest in ion-selective NTs and the degree of difficulty in fabricating such tubes, there are very few related experimental studies. Therefore, initial theoretical studies may prove to be useful to facilitate further experimental studies. In this section we first highlight recent studies that have demonstrated the potential of NTs to nonselectively exclude or conduct ions. Following this, we review research studies that have achieved exclusive selectivity to positively charged (cations) and negatively charged (anions) ions.