Book excerpt: Nanocomposite materials as a special class of nanostructured materials have recently attracted great interest due to their extraordinary mechanical properties as well as thermal stability and oxidation resistance. The unique structure and exceptional properties make nanocomposite materials a possible alternative to traditional polycrystalline materials, which have met their limits in many recent engineering applications. In particular, nanocomposite coatings synthesized by plasma-assisted deposition processes under highly non-equilibrium conditions provide a high potential for new applications as protective and functional coatings in automotive, aerospace, tooling, electronic, or manufacturing industry. This book provides a comprehensive overview of the synthesis of Si-containing hard nanocomposite coatings based on transition metal nitrides by plasma-based thin film processing. It demonstrates the full versatility of these nanocomposites for low Si-containing coatings tailored with superior mechanical properties and novel high Si-containing nanocomposite coatings with extraordinary thermal stability and resistance against oxidation optimized for high-temperature applications. It pays special attention to understanding growth mechanisms of these structures under specific deposition conditions, structure–property relations, and stability of individual constituents to enhance their functionality for various applications.