Jasmonic acid, a linolenic acid-derived cyclopentanone, plays regulatory roles in plant development and responses to environmental stresses, such as mechanical wounding or pathogen attack. The action of JA in regulating plant growth and stress responses often requires the elevation of endogenous levels by de novo synthesis. An especially well studied case has been the synthesis of JA in response to wounding. Jasmonates are synthesised through the octadecanoid pathway, in which linolenic acid is converted into JA by a process that is likely to begin in chloroplasts and end in peroxisomes. Conversion of linolenic acid to 12-oxo-phytodienoic acid in chloroplasts is accomplished through a multi-step enzymatic process involving lipoxygenase, allene oxide synthase and allene oxide cyclase. JA synthesis proceeds with the action of a cytoplasmic 12-oxo-phytodienoic acid reductase and three rounds of β-oxidation that take place in peroxisomes. Most of the enzymes and the corresponding coding genes involved have been isolated and characterised in different plant species. The majority of these genes are transcriptionally activated by wounding and some of them are also activated by JA, allowing feed-back regulation of the biosynthetic pathway.