Two dimensional materials represent a new class of membranes for water-ion and ion-ion separations. With careful tuning, two-dimensional materials can form the basis of highly selective and efficient membranes for water decontamination, energy generation and even biological sensing. Nanofluidic devices made from two-dimensional materials are also excellent testing grounds for complex models of interfacial water structure and confined ionic and molecular transport; they have lead to striking advances in our understanding of solid liquid interactions. Here we present two new devices, based on the two-dimensional material molybdenum disulfide (MoS2), that allow us to achieve Å-scale control of ionic and molecular transport. We both demonstrate that MoS2 membranes are practical ionic and molecular sieves, as well as reveal new transport phenomena for aqueous systems under nanometer-scale confinement. We divide our work into four chapters: in Chapter 2, we present a method to control the interlayer spacing of layered MoS2 membranes. By covalently functionalizing MoS2 monolayers with small molecular pillars, we expand the height of the interlayer gallery from effectively 0 to ∼ 0.6 Å. We show how not only the size but also the chemistry of the functional group determines membrane structure. In Chapter 3, we show how these films can be used to sieve ions from water, and even one ion from another. Using molecular dynamics simulations, we reveal the effect of water layering on the structure of the membranes. In Chapter 4, we present a new procedure for the fabrication (sub)nm pores in few-layer MoS2 membranes in situ. We find that the grain-boundaries in polycrystalline films are excellent nucleation points for electrochemical pore creation. Finally, in Chapter 5, we show how these pores act as ionic sieves, and can differentiate not only monovalent from divalent ions, but also monovalent ions from other from monovalent ions. By comparing transport measurements to molecular dynamics simulations, we show how water dynamics, along with other factors, underlie the separation mechanism.