Metal-organic frameworks or MOFs have witnessed a phenomenal rise owing to a highly tunable synthetic chemistry allowing flexibility in the selection of its constituents, namely metal nodes and linkers. Combined with their superior adsorption and diffusion properties, MOFs have become one of the most promising nanoporous materials for the fabrication of high-performance membranes. Polycrystalline MOF membranes have yielded one of the best gas separation performances, and are expected to replace or partially substitute thermally driven separation processes. In this respect, we present our perspective on the crystal engineering of MOF films that offers control over nucleation and growth of MOFs, film morphology, lattice defects, and therefore the separation performance of the resulting MOF films. © 2018 American Chemical Society.