Covalent organic frameworks (COFs) based on reticular and dynamic covalent chemistry are porous materials with uniform and modifiable pore size, high specific surface area, and structural designability that have attracted widespread burgeoning in membrane separations because of lower mass transport resistance and precision sieving. This critical review focuses on recent advances in COF topology design (two- and three-dimensional COFs) toward membrane building, crucial physicochemical properties of COF-based membranes, synthesis/fabrication methods for COF-based membranes, and state-of-the-art applications of COF-based membranes in sustainable ionic/molecular separations. The perspectives in this fascinating field are discussed in terms of opportunities and challenges for next-generation COF-based membranes for sustainable development.