We develop biomimetic nanoporous membranes for controllable mass transport and separation. There are two types of materials we are interested in: 2D polymers (2DPs) and microporous polymers. Both materials enable the chemical design of nano-scale or Angstrom-scale pores on the molecular level. To synthesize ultrathin membranes, we develop interfacial synthesis approaches to synthesize high-quality (i.e., free of pinholes, crystalline, etc.) films with controlled thickness. For example, we recently developed a new synthetic method, called laminar assembly polymerization (LAP), to grow uniform monolayer 2DPs. This method is compatible with many different molecular precursors and can be applied to synthesize various ultrathin membranes that are inaccessible by other synthetic approaches. These membranes not only provide an ideal system for understanding fundamental transport phenomena, but also represent a new class of semipermeable membranes for water desalination, nanofiltration, and gas separation.