Recent research in livestock farming has focussed on condensed tannins (CT) in feeds, because of their potential benefits for ruminant nutrition and health and for improving the quality of animal products. In the past, all tannins were mistakenly described as ‘anti-nutritional factors’. More recent studies have, however, demonstrated that CT can generate useful benefits for animal welfare and production. Unfortunately, most studies involving CT and animal feeds have been carried out ignoring the great structural diversity that exists among plants and have led to contradictory findings. Plants produce a wide range of different tannins, and tannin composition can differ enormously between plant species, varieties and plant parts. Therefore, investigations into the beneficial or detrimental effects of tannins need to consider all aspects of their structural variation in order to allow conclusions that are scientifically sound. The scientific community urgently requires sound data in order to select plants with desirable tannin properties and to generate sustainable, high quality animal products. This research project seeks (1) to analyse tannin contents and structures in a model plant species (Salix spp.) within a unique and genetically well-characterised germplasm collection; (2) to investigate relationships between tannin structure and their biological effects for ruminant production and health; (3) to identify the genetic traits that control tannin structural features and their bioactivity. If successful, this project will generate a scientific breakthrough in terms of tannin traits that should be evaluated and selected in future research programmes in order to improve the sustainability and quality of ruminant production systems. Results from this research will contribute to developing a basis for improved and more sustainable feeding systems for ruminants in the EU and facilitate application of the results to other CT-rich plants.