Research on metal-organic frameworks (MOFs) based membranes has gained a lot of attention in the last decade thanks to the unique and highly versatile chemistry of MOFs, which allows one to synthesize a variety of structural dimensions, topologies, pore sizes, pore shapes, functional groups, and chemical environments. For membrane application, the field is rapidly progressing, with the focus shifting from the synthesis of pinhole-free polycrystalline films to the synthesis of ultrathin MOF films on scalable support to surpass the performance of polymeric membranes. In this research update, we review promising methodologies for the synthesis of ultrathin MOF membranes. We then discuss the application of these ultrathin MOF films in gas separation, ion transport and ion-ion separation, and desalination. We then provide our perspective on opportunities and challenges for the future development of ultrathin MOF membranes.