Managing the nitrogen (N) supply in agroecosystems is a critical issue to ensure sufficient food production while preserving the environment. Intercropping is known to be a sustainable means of reducing the use of N-fertilizers. However, managing the N supplied to a crop while optimising the use of N by different species in a mixture remains a challenge. This is partly due to the difficulty of assessing the N requirements of each component in the mixture separately in order to correctly drive the performance of the whole system. The methods used to assess plant N nutrition status were initially developed for sole crops. In this review, i) we present different approaches adapted from these previous works to determine the N nutrition status of crops in intercrops; ii) we then compare the predictions of some of them, based on a European database of experimental results regarding cereal–grain legume and perennial grass–legume mixtures; and iii) we discuss the interests and limitations of current indices for mixed crops. We conclude that no single approach is currently valid to assess the N nutrition status of crops within the entire range of possible mixtures explored by farmers. However, some indices based on the N dilution theory perform well regarding the most common situations of balanced binary intercrops. Further research is now needed to properly address the cases of complex mixtures and severely shaded crops.