|In the ACS Nano article “How the Shape and Chemistry of Molecular Penetrants Control Responsive Hydrogel Permeability” we uncover the molecular principles of permeability and selectivity in hydrogels. The permeability of small molecules (drugs, toxins, reactants, etc.) through hydrogels is a central property in the design of soft functional materials in biomedical, pharmaceutical, and nanocatalysis applications. Dense hydrogels are extremely selective, owing to a delicate balance between the partitioning and diffusivity of the molecules. These properties are sensitively tuned by the molecular size, shape, and chemistry, leading to vast cancellation effects, which nontrivially contribute to the permeability.