All-organic flow batteries are poised to meet cost and performance targets for grid-scale electrochemical energy storage. One of the outstanding challenges in these systems concerns unwanted active material crossover, where small organic charge-storing (charge-releasing) molecules are allowed to traverse a non-selective membrane separating the positive and negative electrode compartments. Crossover leads to a shuttling process that both reduces energy ecfficiency and contributes to capacity fade. I will discuss our recent progress in understanding the molecular basis of active-material crossover for redox-active organic molecules across microporous polymer membranes with controlled pore size and pore chemistry. These membranes are based on a modular platform, namely polymers of intrinsic microporosity, or PIMs. These membranes uniquely feature sub-nanometer pores and are highly permeable. Using them, it now becomes possible to tailor both the membrane and the redox-active organic molecules to minimize the impact of crossover on cell performance.