With nanoscale pores, high porosity, narrow pore size distributions, and tunable chemical and mechanical properties, block copolymers hold tremendous potential as robust, efficient, and highly selective separation membranes. Recent work by Yang et al. illustrates how block copolymers can be employed in the preparation of membranes for use in controlled, long-term, protein-delivery devices. Their work demonstrates that highly efficient and tunable separations are possible with block copolymer membranes. Although significant progress over the past 10 years has advanced the quality, efficacy, and applicability of such membranes, more work is required before benefits are realized for other demanding applications such as water purification.