Grasses (Poaceae) and legumes (Fabaceae) represent the two most important botanical families for food and feed production. They are frequently grown as mixtures (i.e. intercropped simultaneously in the same field) to take advantage of their different ecological and agronomical requirements as well as of their nutritional complementarities. Yield, quality and environmental efficiency of the grass-legume mixtures directly depend on the contribution of the legume in the mixture at harvest. The control of mixture change however remains a challenging task. This article aims at examining how the interactions between plant traits, environment (and more particularly resource availability) and crop management influence productivity and species composition of grass-legume and cereal-grain legume mixtures of temperate origin. The role of developmental processes and resource-based interactions (competition and facilitation processes) in the occurrence of plant complementarities are more particularly discussed. In the last section, the effects of different management practices on mixtures are presented with particular references to different cropping systems (forage, grain, and bio-fuel production).