Cross-linked lyotropic liquid crystal (LLC) assemblies represent a new class of polymer materials for membrane applications. These materials are formed by the phase-segregation and self-assembly of polymerizable amphiphiles in water into condensed ordered ensembles that can be cross-linked in situ with retention of microstructure. The resulting LLC polymer networks have ordered, nanometer-scale aqueous and cross-linked organic domains, which can be used to affect gas solubility and diffusivity through the polymer to help separate molecules via the solution-diffusion mechanism. The open aqueous domains can also be used for pore transport and size exclusion with resolution on the molecular size level. The use and application potential of cross-linked LLC assemblies as gas separation membranes, selective vapor barrier materials, and water nanofiltration and desalination membranes are presented.