Ion transport across lipid bilayer membranes is a tightly regulated process in living organisms, carried out by highly specific pumps and channels. Given the severe implications of genetic diseases that cause dysregulation of ion channels, known as channelopathies, much attention has been directed towards the development of synthetic analogues to natural transporters. Among these are artificial ion channels, in which the self-assembly of small molecular components is used to form pores in the membrane, and mobile carriers, which are small molecule ion receptors (ionophores) that shuttle ions across the membrane. The structural design of mobile ion carriers is governed by their functional requirements: they must bind hydrophilic ions at the membrane-aqueous interface while also transporting them across lipophilic membranes. This requires a delicate balance of directional non-covalent interactions for ion recognition, lipophilicity of the complex, and molecular size and geometry. Transporters that successfully integrate these characteristics could serve as biological probes for interrogating natural ion transport processes, as well as underpinning future therapies for channelopathies, such as those resulting from chloride channel mis-regulation, including cystic fibrosis. However, to move beyond experiments in vesicle-based models into cells, these transporters must also demonstrate a high degree of selectivity for the target ions. For example, chloride transporters typically also bind and transport other monovalent anions, including hydroxide. Moreover, their anion binding groups often comprise highly acidic N-H hydrogen bond donors, which can become reversibly deprotonated and thus facilitate proton transport. These mechanisms of proton and hydroxide transport can contribute to the dissipation of pH gradients, leading to cytotoxicity. Whilst this may be advantageous for inducing apoptosispotentially aiding cancer therapy -this lack of selectivity limits the utility of chloride transport in other applications (Figure ). Figure . a) Anionophore selectivity for Cl -over OH -transport is crucial for many biological applications. 1,2,5,10 b) Previous work exploiting (top) halogen bonding and (bottom) encapsulation to enhance Cl -over H + /OH - selectivity, S. c) The family of macrocyclic Cl -transporters reported in this work that combine halogen bonding and anion encapsulation.