Nitrogen (N) is the most abundant mineral elements in plants, and the application of inorganic N fertilizer makes huge contribution to the crop production and global food security. However, low N use efficiency (NUE) and overapplication of N fertilizers causes ever-growing environmental problems. Understanding the molecular mechanisms of N sensing and signalling in plants will provide molecular basis for NUE improvement of crops. Forward genetics screening and functional analysis have characterized the NRT1.1-NLP centered N signalling pathway at the cellular level. With the incorporation of systems biology approaches, a preliminary N regulatory network has been delineated. Meanwhile, long-distance N signalling has also been unveiled at the whole plant level. This review highlights most recent understanding of the N signalling network in plants, and also discusses how to further integrate hierarchical regulation of N signalling in plants.