Conjugated linoleic acid (CLA) is a naturally occurring fatty acid that is produced by a bio-hydrogenation process in the rumen, and thus is present in dairy products and ruminant meat. In this case the predominant isomer formed is 9cis,11trans. However, CLA includes 28 positional and geometrical isomers, of which only 9cis,11trans and 10trans,12cis have thus far been proven to possess biological activities. Both of these CLA isomers have been shown to undergo elongation and desaturation processes similar to those that occur with linoleic acid, maintaining the conjugated diene structure. There are evidences supporting the hypothesis that CLA metabolism may interfere with eicosanoid formation. Other metabolites with 16 carbon atoms (conjugated 16:2 and 16:3, which are probably derived from peroxisomal beta-oxidation of CLA and its metabolites, respectively) have been detected. This suggests an efficient metabolism of CLA and its metabolites in peroxisomes, which might be linked to their capacity to activate peroxisome proliferator-activated receptors.