Directed delivery of mesoscaled cargofor example, nanocrystals, proteins, or nucleic acidsto cells using polymer vectors impacts numerous biomedical fields. We introduce here the concept of dynamic complementarity as a simple, yet powerful approach to control the rate of mesoscaled cargo dissociation from colloidal polymer vectors once inside the cytosol. By tuning the degree of electrostatic reciprocity between the polymer vector and its cargo, it is possible to both deliver and release large cargo in live cells in a controllable manner over both long and short periods, pointing to a highly modular materials platform with molecularly tailored properties suited to task. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 256-264