This dissertation aimed to improve the understanding of the reactions of N-containing compounds during hydrothermal carbonization (HTC), the synthesis of target-oriented N-hydrochar (N-HC), and the potential of N recovery in HTC. The results of this work showed that recovery of N to the HC during HTC mainly depends on the composition of the feedstock before HTC. Inorganic-N is recovered to the HC via sorption, except for NHN4-N which can be also chemically incorporated into the structure of the HC. Moreover, the availability of the carbon network in the HC is a key factor for N recovery to the HC. It transpires that N-containing compounds have a significant influence on the formation of HC via the solved-intermediates pathway during HTC. The pH value of the reaction environment during HTC can guide the Maillard reaction (MR) toward the formation of N-HC or oil-like materials. N-containing compounds showed a notable influence on the morphology and composition of N-HC, this influence included the formation of heterocyclic N in the HC, reducing the O content on the surface of HC, and affecting the formation of spherical HC. The results exhibited a promising performance of HTC for the synthesis of N-enriched carbon materials (NCM) as electrode materials in electrochemical double-layer capacitors (EDLCs). HTC enabled the incorporation of N into the bulk of the NCM and improved the overall capacitance of the produced NCM. The outcome of this dissertation helps to support further research toward a sustainable conversion of biomass to a target-oriented N-HC for previously identified applications.