Secondary metabolites represent a diverse group of natural products. Although precise numbers are, at best, an estimate, at least 100,000 different compounds of natural origin have been described, of which at least 80,000 of those are derived from plants. It is likely that many more than these will be isolated in the future, as increasing numbers of novel plant species are being identified and studied for their potential use in the pharmacological, medical and agricultural industries. The increased interest in phytochemicals in animal diets has been spurred on by the EU ban on the use of ‘in feed’ antibiotics, the removal of animal proteins from the diet and thus the increased variety and inclusion levels of vegetable protein sources.