The Poisson-Boltzmann theory, used almost exclusively in the description of space charge zones in solid electrolytes and mixed conductors, is strictly valid only in the dilute limit. Although it is useful for dilute systems, a host of experimental results in different materials, from different groups using different techniques, confirm the failure of the theory in concentrated systems, in that the thickness of charge accumulation zones are consistently an order of magnitude larger than that predicted by Poisson-Boltzmann. Poisson-Cahn adapts the Cahn-Hilliard theory of inhomogeneous systems to systems containing charged particles near surfaces and interfaces. The result is a theory that reproduces the experimental findings on space charge in concentrated systems, while preserving the dilute-case limiting behavior predicted by Poisson-Boltzmann. An overview of the theory along with applications to grain boundaries and surfaces in pure and acceptor-doped ceria will be presented, along with comparisons to experimental results in those systems.