Grass is a complex crop and its value for agriculture must be assessed in terms of the quantity and quality of downstream livestock products (milk, meat and wool). In addition to being a natural low-cost feed for ruminants, grassland protects soil and water resources and enhances the landscape. Temperate grasslands support a major share of the world's milk and beef production but now there is increasing emphasis on sustainability in livestock farming systems rather than maximizing outputs. The economic sustainability of livestock producers in many developed temperate regions is increasingly linked to production from fewer animals giving a higher quality product. A key element in the efficiency of all grassland systems is to optimize the protein/energy balance of forage and value it in a similar way to other livestock feeds. Grasses are rich in energy comprising structural and non-structural carbohydrates while forage legumes are rich in protein. The main forage legumes used in Europe are white and red clover, and lucerne, which form an essential part of sustainable farming systems because of their high nutritive value and ability to fix nitrogen. The productivity of grasslands containing legumes generally reflects the amount of N fixation, which is 65–280 kg/ha/year in W. Europe. Animal production from white clover-based pastures in Europe can be 0·8 that obtained from grass pastures fertilized with 400 kg N/ha/year, and that from lucerne and red clover can be equivalent to animal production from such pastures.