The continuous evolution of advanced nanomaterials has redefined possibilities for sustainable energy, environmental, and biomedical technologies, with nitrogen-doped graphene (N-G) standing out as one of the most adaptable and multifunctional carbon-based materials. This chapter presents a comprehensive overview of the diverse applications of N-G, highlighting its tunable electronic structure, defect rich surface, and strong interfacial interactions that enable exceptional performance across energy generation, water treatment, and biomedical systems. In water electrolysis, N-G serves as both a metal free catalyst and a support for hybrid electrodes, effectively catalysing hydrogen and oxygen evolution reactions, reducing overpotentials, and enhancing durability. It also functions as a co-catalyst in photocatalytic hydrogen generation, promoting charge separation and stability in semiconductor systems. In water purification, N-G based membranes and aerogels provide excellent hydrophilicity, permeability, and antifouling characteristics, while N-G hydrogels enable efficient solar driven steam generation and desalination. Biomedical applications of N-G and its quantum dots include photothermal and photodynamic therapies, biosensing, bioimaging, and antimicrobial coatings, where tunable surface chemistry enhances biocompatibility and functional selectivity. Beyond these areas, N-G finds use in tribology, corrosion protection, electromagnetic interference shielding, gas sensing, and thermal management, as well as in flexible and wearable sensors for real time monitoring and human–machine interfaces. The chapter concludes that N-G’s versatility, scalability, and multifunctional nature position it as a cornerstone material for next generation technologies addressing global challenges in clean energy, water security, healthcare, and smart devices.