Ion channels are pore-forming proteins that allow the diffusion of charged particles through the plasma membrane. They play an important role in many cellular functions and are also vital drug targets. One key characteristic to their proper function is their ion selectivity, yet, the mechanisms underlying this are still not fully understood. This thesis explores the ion selectivity in the Epithelial Sodium Channel/Degenerin (ENaC/DEG) family, a group of voltage-insensitive sodium-selective trimeric channels.Our initial systematic review identified that the selectivity filter of this family has been characterised as the ‘G/SxS’ belt, but other residues such as the amiloride binding site and the conserved ‘HG’ motifs are also important for ion selectivity. Our work then focuses on these conserved residues in Acid-Sensing Ion Channel 1 (ASIC1). In addition to confirming the cation selectivity, our umbrella sampling simulations revealed that the binding site for Na⁺ consists of the glycine from the `GAS' belt and the histidine from the `HG' motifs, redefining the selectivity filter as a combination of both the `G/SxS' and `HG' motifs. Furthermore, ion coordination analysis revealed that Na⁺ permeates the channel in a partially hydrated state. Following this, we investigated the role of water in ion selectivity by performing electrophysiology experiments with deuterium oxide. Our study showed that heavy water substitution affects the kinetics, the conduction and the selectivity of ASIC1, with viscosity only affecting the kinetics. To further explore this isotopic effect, we performed more simulations with Na⁺, K⁺ and Li⁺ in normal and heavy water conditions using classical and quantum theory. Our results suggest that the hydration energies influence the conduction and permeation of the ions.Altogether, our work indicates that ion selectivity in ASIC1 is correlated to the physical and chemical properties of the ions and their first hydration shell, bringing new insights towards understanding the mechanisms of ion selectivity.