Intercropping – growing two or more crop species on the same field at the same time– is considered as one of the practices to achieve the goals of sustainable intensification of agriculture. However, as intercropped species may also compete, the challenge of intercropping is to ensure that all crops have sufficient resources at the phenological stages of greatest need, so that competition is minimised and plants are allowed to invest their energy in growth and biomass allocation rather than wasting it to compensate for, or overcome, the harmful effects of competition. The aim of my PhD thesis was to highlight the competition and complementarity interactions in the above and belowground plant parts of cereals (triticale and barley) and field bean intercropped for forage and grain production. The response of the species grown as sole crops and intercrops in the field and in growth boxes, was analysed in terms of forage and grain biomass, N and P yield, and in the changes in plant morphology, partitioning of resources, yield components and functional traits. A three-year field experiment was set up with two intercrop row ratios (1:1 and 2:1 C:Fb) and five sole crops (C50, C70, C100, Fb60 and Fb100), and two fertiliser levels (0 and NP). In parallel, a set of additional experiments was carried out to study above and belowground interactions, and potential ecosystem services in terms of weed development, soil moisture sparing, soil biological activity and insect biodiversity. Moreover, the influence of N fertiliser on P uptake, the facilitation of Zn and Fe uptake in seed, and whether crop rotation was necessary to maintain intercrop performance over time were assessed. The yield advantage of intercropping over sole crops was appreciable, at the vegetative stage, independently of row ratio and fertiliser supply; at the flowering harvest, only under unfertilised conditions; and, at maturity, only with the unfertilised 1:1 IC row ratio. At flowering, the forage yield of intercrops was not affected by fertiliser supply, whereas nitrogen fertilisation improved seed production at maturity. These trends clearly demonstrated that the increase of interspecific competition progressively reduced the benefits of complementarity and facilitation deserved by intercrops. Moreover, investigations performed in semi-controlled conditions and in the field with both triticale and barley, demonstrated that when mineral nitrogen was available in the soil, field bean invested energy in root elongation rather than in supporting nodule growth and activity, and competed with cereals for mineral nitrogen uptake. This suggests that, in cereal-field bean intercrops, the niche complementarity for N uptake is greatly dependent on mineral N availability, and that field bean shaped root architecture in response to the type of N source. It was concluded that both field bean and the two cereals tested were able to adapt to intercropping by modifying their morphology to overcome competition and showed high replacement ability, which means that, although the proportion of the species in the intercrop varies, the overall production of the system remains constant or even increases. In field bean, early competition decreased the number of stems during the vegetative phase as well as the number of pods per stem at the beginning of the reproductive phase, while later competition decreased, in the order, pod growth, pod fertilisation and, finally, seed filling. Regarding the partitioning of biomass in stems, leaves and pods, in all years and with both seed densities, biomass decreased when field bean was intercropped with the cereal. Moreover, the specific stem length – a functional trait used to estimate shoot plasticity in response to competition – was more than 30% higher in intercrops than in sole crops. In barley, spikes showed the largest size in the low-density sole crop (C50), thus demonstrating that both intra (C100) and interspecific interactions (1:1 IC) adversely affected spike development. The morphology of barley spikes showed that competition reduced the number of spikelets, revealing that competition started early, between tillering and stem elongation and, in unfertilised conditions, it also impaired spikelet differentiation. Finally, our results showed that three years of continuous intercropping reduced the proportion of barley, because of lower culm biomass and lower number of spikes, especially in unfertilised conditions. This research proved that cereal-field bean intercropping might also deliver several ecosystem services. Compared to sole crops, intercrops greatly reduced weed development in both fertilised and unfertilised conditions, maintained higher water content in soil in drought periods, and increased C sequestration in the soil. Moreover, field bean flowers were found to attract parasitoids (braconids) active against insects noxious to cereals.