In 1999, Freeman published a visionary article that proposed a molecular basis for the trade-off between permeability and selectivity for polymeric gas separation membranes, which is often codified as an “upper bound”. This work has had major impacts in the gas separation membrane community and is paralleled by developments in the water purification membrane literature. A common theme between both communities is optimizing free volume or pore size distributions in polymer membranes to maximize separation performance. In guiding future development in the field, we identify the need to develop isoporous ultrafiltration membranes and highlight the potential for block copolymer self-assembly to achieve direct access to “structure by design” without requiring complex optimization of phase inversion processes.