This review article discusses the multi-scale physics of bipolar membranes (BPMs) in electrochemical processes and outlines design principles for advanced BPMs. BPMs are ion-conductive polymers with two layers of fixed charges, often with a catalyst layer between them. They enable control of ion concentrations and fluxes in electrochemical cells, and their performance is influenced by thermodynamics, transport phenomena, and chemical kinetics. The interactions within BPMs lead to emergent structure–property–performance relationships that guide the development of high-permselectivity, durable, and efficient BPMs. BPMs operate in two modes: forward and reverse bias. In forward bias, cations and anions are transported towards the AEL/CEL interface, where they recombine. In reverse bias, electric fields and catalytic effects drive the breakdown of polarizable species into cations and anions that are transported into the CEL and AEL. The fixed charges on the ionomer limit crossover of co-ions through the BPM by electrostatic repulsion, while counter-ions are selectively partitioned and transported. BPMs can control and sustain different local chemical environments, enabling the interconversion of chemical- and electrical-potential gradients. Since their invention in 1956, BPMs have been used in various applications, including the food industry and production of acid and base. Recent advancements in ionomers, low-cost renewable electricity, and the recognition of the value of controlling local reaction environments have led to a renaissance in BPM development. The development of advanced interfacial catalysts has significantly reduced BPM energy requirements, enabling performance close to the thermodynamic minimum even at high current densities. The chemical and physical properties of ion-conducting polymers (ionomers) are crucial for BPM performance. Ionomers are classified based on the type of counter-ion they conduct. The chemical structure of ionomers consists of long-chain polymer backbones with fixed-charged groups that enable selective uptake and transport of counter-ions. The ion-exchange capacity (IEC) is a measure of the moles of fixed-charge groups per unit mass of polymer. Structural factors also control ion transport, including nanostructure, confinement effects, and mesoscale morphology. The governing physics of BPMs involve thermodynamics, transport phenomena, and chemical kinetics. The thermodynamic coupling between species influences the behavior of all components in the system. Ion and solvent partitioning between the external solution phase and the membrane phase can be described by electrochemical equilibrium. The speciation between absorbed species and solvent is critical in BPMs, particularly the effects of water dissociation due to its prevalence as a solvent. Species transport in BPMs is dictated by the interplay between non-equilibrium reaction and transport processes. These processes include the transport of co- and counter-ions, transport of solvent to and from the BPM junction, dissociation of species, and homogeneous reactions. Understanding species transport is necessary to resolve BPM performance, including theThis review article discusses the multi-scale physics of bipolar membranes (BPMs) in electrochemical processes and outlines design principles for advanced BPMs. BPMs are ion-conductive polymers with two layers of fixed charges, often with a catalyst layer between them. They enable control of ion concentrations and fluxes in electrochemical cells, and their performance is influenced by thermodynamics, transport phenomena, and chemical kinetics. The interactions within BPMs lead to emergent structure–property–performance relationships that guide the development of high-permselectivity, durable, and efficient BPMs. BPMs operate in two modes: forward and reverse bias. In forward bias, cations and anions are transported towards the AEL/CEL interface, where they recombine. In reverse bias, electric fields and catalytic effects drive the breakdown of polarizable species into cations and anions that are transported into the CEL and AEL. The fixed charges on the ionomer limit crossover of co-ions through the BPM by electrostatic repulsion, while counter-ions are selectively partitioned and transported. BPMs can control and sustain different local chemical environments, enabling the interconversion of chemical- and electrical-potential gradients. Since their invention in 1956, BPMs have been used in various applications, including the food industry and production of acid and base. Recent advancements in ionomers, low-cost renewable electricity, and the recognition of the value of controlling local reaction environments have led to a renaissance in BPM development. The development of advanced interfacial catalysts has significantly reduced BPM energy requirements, enabling performance close to the thermodynamic minimum even at high current densities. The chemical and physical properties of ion-conducting polymers (ionomers) are crucial for BPM performance. Ionomers are classified based on the type of counter-ion they conduct. The chemical structure of ionomers consists of long-chain polymer backbones with fixed-charged groups that enable selective uptake and transport of counter-ions. The ion-exchange capacity (IEC) is a measure of the moles of fixed-charge groups per unit mass of polymer. Structural factors also control ion transport, including nanostructure, confinement effects, and mesoscale morphology. The governing physics of BPMs involve thermodynamics, transport phenomena, and chemical kinetics. The thermodynamic coupling between species influences the behavior of all components in the system. Ion and solvent partitioning between the external solution phase and the membrane phase can be described by electrochemical equilibrium. The speciation between absorbed species and solvent is critical in BPMs, particularly the effects of water dissociation due to its prevalence as a solvent. Species transport in BPMs is dictated by the interplay between non-equilibrium reaction and transport processes. These processes include the transport of co- and counter-ions, transport of solvent to and from the BPM junction, dissociation of species, and homogeneous reactions. Understanding species transport is necessary to resolve BPM performance, including the