Nitrogen is one of the most important mineral nutrient elements for plant growth and development, playing an essential role in the whole process of plant life. Nitrogen assimilation is a central link for plants to utilize nitrogen and also a factor in low nitrogen use efficiency in plants. It includes two types: assimilation of nitrate (NO3 ^–) and ammonium (NH4 ⁺); the latter is the critical step in the process of nitrogen assimilation. The source of NH⁴
+ during ammonium assimilation can be divided into 2 types—primary assimilation and secondary assimilation—but both proceed in the glutamine/glutamate (GS/GOGAT) pathway. Ammonium assimilation requires much energy resources but also consumes abundant carbon skeletons, so it is strictly regulated at different levels including transcription, post-transcription, and post-translation. We review the current progress in study of ammonium assimilation and its regulatory mechanism in plants.