The adsorption of functional organic molecules, such as pharmaceuticals, personal care products, herbicides, insecticides, etc., in high-silica and all-silica zeolites is relevant in the context of different applications: On the one hand, these hydrophobic adsorbents could be employed in the removal of emerging organic contaminants from wastewaters. Promising results have been obtained, for example, for the use of faujasite-type zeolite Y (FAU framework [2] ) to remove sulfonamide antibiotics, other drugs like carbamazepine, and the personal care product triclosan, for high-silica mordenite (MOR framework) as adsorbent for various pharmaceuticals and other organic contaminants, and for zeolite beta (BEA framework) in the removal of drugs, pesticides, and perfluoroalkyl substances. On the other hand, high/all-silica zeolites could also find use as carrier materials in the controlled delivery of drug molecules or for the encapsulation of organic UV filters, enhancing their UV filtering activity while at the same time preventing undesired release into the environment. Further potential applications of zeolites involving the adsorption of sizeable organic molecules from the liquid phase include the recovery of valuable fractions from complex mixtures, e.g., in the processing of rapeseed, and the use of zeolites as solid sorbents in (micro)extraction. Given the vast number of potentially interesting organic species, and the multitude of zeolite frameworks that are available in highly or purely siliceous form (about 70 zeolite frameworks have been synthesised as all-silica zeolites to date ), it is clear that experimental investigations of the adsorption properties are feasible only for a small fraction of the theoretically possible adsorbent-guest combinations. To narrow down the selection, atomistic simulations can be employed to gauge the affinity of different zeolites towards a species of interest. Recent work showed a good correlation between zeolite-guest interaction energies and experimentally observed removal efficiencies for 21 organic contaminants in MOR-and FAU-type zeolites. Interaction energies were computed using a fairly simplistic approach based on force field (FF) Monte Carlo (MC) simulations and subsequent energy minimisations with the DREIDING FF, indicating that even relatively crude simulations could be very useful to identify adsorbent-guest combinations of interest. Other authors have employed FF-based molecular dynamics (MD) simulations to study the diffusion of pharmaceuticals through zeolite structures in the context of drug delivery investigations. A crucial aspect in this regard is the suitability of the FF parameters to represent the relevant interatomic interactions in the system, especially the interactions between zeolite host and adsorbed molecules. For gas phase adsorption, a validation against experimental adsorption data is relatively straightforward, and high-quality FFs that accurately reproduce adsorption isotherms for gases like CO2, CH4, N2, or O2 in all-silica zeolites have been developed. For larger organics that are adsorbed from the liquid phase, the situation is more complex, as a direct comparison to liquid-phase adsorption isotherms not only requires a reasonably accurate description of the interactions between zeolite and organic guest, but also of interactions between the organic species and the solvent (typically water). Some progress in this direction has been made, and expanded-ensemble MC simulations have been shown to reproduce liquid-phase adsorption data rather well. Altogether, however, a validation against experimental data remains a very complex endeavour, not least due to the impact of structural defects (usually ignored in the simulations) on the experimental isotherms.