Chemically derived graphene and its derivatives, the thinnest material known to science, are reasonably believed to be very promising candidates for separation membranes. It has been widely demonstrated that graphene-related membranes, including nanoporous graphene and graphene oxide membranes, have great potential for water purification and gas separation. In this chapter, the recent advances on these two graphene-related membranes for application in water purification and gas separations are reviewed. Representative theoretical and experimental work is highlighted, accompanied by a discussion of the separation mechanisms and fabrication methods of these state-of-the-art graphene-based membranes. It is highly expected that more and more researchers will participate in this cutting-edge research field to overcome the key challenges we are currently facing, making these atomically thick two-dimensional membranes a reality in industry.