Vladimir Prelog spent decades decoding the precise three-dimensional arrangements of organic molecules, revealing how the spatial orientation of atoms determines the way a substance behaves. Born in Sarajevo in 1906, he grew up in Zagreb, studied chemistry in Prague, and eventually joined the Swiss Federal Institute of Technology (ETH Zurich) in 1942, where he spent the rest of his career. His central contribution was to stereochemistry, the branch of chemistry concerned with the three-dimensional structure of molecules. Two molecules can have exactly the same atoms connected in the same order but arranged differently in space, like a left hand and a right hand. These mirror-image pairs, called enantiomers, often behave very differently in biological systems: one version of a drug molecule might cure a disease while its mirror image is inert or toxic. The thalidomide disaster of the 1960s made this distinction tragically concrete. Prelog, working with Robert Cahn and Christopher Ingold, developed the CIP priority rules, a systematic notation for describing molecular handedness. These rules became the universal language chemists use to communicate about three-dimensional molecular structure. Before CIP notation, describing stereochemistry was ambiguous and error-prone. After it, chemists worldwide could read molecular "maps" with the same precision as reading coordinates on a globe. He also investigated the stereochemistry of medium and large ring compounds, molecules whose shape and flexibility determine their chemical behavior in ways that smaller molecules don't exhibit. His work on enzyme specificity helped explain why biological catalysts are so selective about which molecules they interact with. He shared the 1975 Nobel Prize in Chemistry with John Cornforth for their combined work on the stereochemistry of organic molecules and enzyme-catalyzed reactions. Prelog died on January 7, 1998, at 91, in Zurich.