Chorismate synthase is the seventh enzyme of the shikimate pathway and is a target for novel antibiotics and herbicides. It converts 5-enolpyruvyl shikimate-3-phosphate (EPSP) to chorismate by cleaving two bonds in an unusual 1,4-trans elimination reaction with no net redox change. The absolute and intriguing requirement for reduced flavin mononucleotide (FMN) for catalysis was, until recently, a mystery. We have obtained kinetic and spectroscopic evidence for the novel radical mechanism shown below that explains both the stereochemistry and the role of FMN. We have also used rationally designed substrate analogues and inhibitors to help elucidate both the catalytic mechanism and the mode of action of compounds with antibiotic and herbicide activities. We have recently obtained evidence that an invariant aspartate side chain is essential for catalysis, consistent with it acting as a base to set up an anionic flavin sequiquinone to accept the proton from the substrate. In addition, two conserved Ser residues appear to be required for a proton relay system between the flavin and the phosphate leaving group. These mutagenesis studies were in collaboration with Peter Macheroux (Graz, Austria).