Nitrogen (N) is the main limiting macronutrient for plant growth and N deficits lead to severe negative effects in
plant metabolism and development. Hence, knowledge about the complex dynamics of plant N nutrition is essential to comprehend plant functioning in natural and managed ecosystems. The availability of N varies widely among these systems, and this has a significant effect on how successful plants compete with soil microorganisms for available sources of N. It is widely accepted that plants incorporate, transport and recycle inorganic and organic forms of N, including amino-N. Uptake of amino-N can be direct via shuttle mechanisms that transport intact amino acids, but amino-N can also be gained through plant-fungal (i.e. mycorrhizal) or plant-bacterial (i.e. rhizobial) symbioses, yet it can also be lost through parasitism. Regardless of origin, once amino-N has entered the plant root, synthesized amino-N compounds play a key role in long-distance transport of N, metabolism, N storage and the regulation of N uptake. This chapter provides an overview how plants, in particular woody plants, acquire, transport and recycle amino-N. Further, it summarizes our current understanding of why organic N is of varying importance for plant nutrition across biomes and also outlines future directions for research that can help to further complete our understanding of the role of amino-N in ecosystem functioning.