Metal-organic frameworks (MOFs) have been emerging as promising multifunctional materials and have shown particularly useful applications for gas storage and separation. We have briefly outlined the early development of this very active research field to provide us a clear picture on what are MOFs and how the research endeavor has been initiated and explored. Following that, we have demonstrated why MOFs are so unique for gas storage and separation: high porosities, tunable framework structures, and immobilized functional sites to fully make use of pore space for gas storage, to optimize their sieving effects, and to differentiate their interactions with gas molecules. Finally, we have provided a perspective on further development of porous MOFs for gas storage and separation.