From olive leaves and olive oil mill waste waters, which are produced in large quantities in the Mediterranean countries as well as in other parts of world where olive oil is produced, the naturally occurring polyphenols were identified and the main of them were isolated by chromatography and spectroscopically characterized. The biological effects of these compounds and some of their derivatives were tested, in order to assess their environmental impact and evaluate their use as by-products for agriculture and industry. Chromatographic and spectroscopic analytical methods were developed to detect them in vegetable raw materials mentioned above. In addition, a method using laccase to detoxify polluting polyphenols was developed, as well as different β-glucosidase immobilisation techniques were experimented, in order to produce from oleuropein its aglycone and glucose, which could be commercially interesting products. Lastly, the possible exploitation in agriculture and industry of certain polyphenols is discussed.