Víctor M. Rivilla, Miguel Sanz-Novo, David San Andrés

The increasing detection of new molecules in the interstellar medium (ISM) shows that stereoisomerism is a fundamental contributor to interstellar molecular complexity. This work presents the first comprehensive overview of interstellar stereoisomerism. A total of 16 stereoisomeric pairs have been identified (13 conformational and 3 geometric), spanning molecules with 5-12 atoms and energy separations from 10 K to 2667 K. They were observed across diverse astrophysical environments with kinetic temperatures ranging from low to high values (7.5-300 K). The observed stereoisomeric ratios (OSR) - defined as the column density ratio of the higher-energy isomer divided by that of the lower-energy isomer - vary widely (0.009-4). While systems with small energy differences (1.2 kcal/mol) in hot environments (> 100 K) generally follow thermodynamic expectations (often assisted by tunneling-driven interconversion), many stereoisomers - particularly those in cold clouds or with larger energy separations - exhibit abundances far exceeding equilibrium values. This demonstrates that thermodynamics alone cannot explain interstellar stereoisomerism. Instead, stereoselective formation/destruction pathways (in the gas phase and/or in the surface of dust grains), photoisomerization, and chemical rearrangement during desorption must play a dominant role. Stereoisomeric ratios thus provide powerful constraints on interstellar chemical pathways, and about the physico/chemical conditions of the ISM. This review highlights the need for stereochemistry-sensitive astrochemical models. Progress in this field requires expanded laboratory spectroscopy of higher-energy stereoisomers, dedicated quantum chemical studies of isomerization processes, and the explicit inclusion of stereoselective chemistry in chemical networks.