Poultry producers have been cautious in the application of probiotics, due to mixed experiences of their effectiveness and lack of knowledge regarding the scientific basis for their modes of action. Prebiotics have been more readily accepted because they can be added without any significant change to traditional practices. Recent research has increased our knowledge of the effects of both types of treatment, their possible modes of action, and the strengths and limitations of their use. We are beginning to understand why some products may give variable outcomes and what may be done to further validate the performance of existing products and develop a new generation of more reliable and effective probiotics and prebiotics.