The relationship between specific diversity and the productivity of pastures (both leys and permanent pastures) has lately been often studied, from the agricultural as well as from the ecological point of view. The present bibliographic synthesis should facilitate the comprehension of the factors involved in this relationship. The studies analysed show that a greater specific diversity and a larger number of functional groups tend to bring about an increased above-ground bio-mass, particularly under conditions of poor fertility. These studies show also the predominant influence of certain species in the increased productivity, and that of the environment and of farmers' practices. A large number of mechanisms are involved in the workings of multi-specific plant communities : competition, complementarity, and facilitation, and also, in the case of grass - legume associations, the nutritional interactions, which are of primary importance; lastly, there is the vegetation dynamics and especially the renewal of seedlings. The analysis of this relationship opens new fields of research. It also demonstrates the importance of the different ways at the farmers' disposal to optimize the interactions among the plant species.