In the previous chapters, we have largely focused on the role of transmission between CNTs and its effect on the conductance of the network as a whole. In this chapter we investigate a separate mechanism, that of charge doping due to adsorbed water molecules, and consider its effect on the conductivity of individual CNTs.The effect of water on the electrical properties of carbon nanotubes (CNTs) has been controversial for some time. Theoretical calculations have suggested that water dopes CNTs, predicting a strong dependence of CNT conductivity on humidity. The theoretical evidence for doping in this system has been questioned, however, and experimental studies have not been able to clearly confirm or refute this model. In this chapter, we propose that the theoretical methods used in previous calculations are indeed insufficient. We investigate in detail the conditions that must be satisfied for charge doping to occur, and provide a theoretical method, based on large-scale density functional theory calculations, to identify the presence of charge doping. Key to our analysis is the isolation of electrostatic effects, which we do using a novel application of maximally-localised Wannier functions. Our findings lead us to conclude that water does not dope CNTs.Parts of this chapter have been published [1].