Abstract: Nitrogen is one of the essential elements for plant growth and development. External application of nitrogen promotes plant growth, but excessive fertilization can induce a host of problems on soil and water pollution, ozone layer depletion, and agricultural production cost escalation. In nature, to efficiently utilize the resource, plants have evolved mechanisms that regulate the absorption of the critical nutrient transported from the land. This article reviews such fundamental natural functions, in addition to the processes of NO₃⁻N, NH₄⁺-N, and organic nitrogen assimilations as well as the roles of transcription factors and miRNAs in response to nitrogen fertilization. Based on the information, improvements on the nitrogen use efficiency of plants, agriculture productivity, and breeding and cultivation of new crop varieties could be studied and realized.