Mixed ionic–electronic conductivity of metal halide perovskites is a critical factor determining their application in optoelectronics, as well as the emerging field of optoionics. In particular, ionic migration in response to voltage bias and light contributes to the operational instability of halide perovskite materials and devices. A promising strategy to control ion migration is dimensional reduction, achieved by incorporating bulky organic cations between inorganic slabs to form two-dimensional or layered hybrid halide perovskites. In this perspective article, we discuss the mixed conductivity of layered halide perovskite materials and the impact on the operational stability of perovskite optoelectronics and emerging applications.