Important hard phases are included in the quaternary compositional system Si-N-C-B. This paper reviews ternary amorphous and crystalline phases in the system Si-N-C and deliberates on the issue of stability of the binary C3N4, a hypothetical phase harder than diamond, and instability of nitrides in general. There is a tendency for nitrogen atoms to agglomerate and be released as nitrogen molecules. Stabilization of CN radicals can be achieved through ternary phases: carbonitrides metal-C-N. Ternary Si-N-C phases have been synthesized by pyrolysis of polyorganosilazanes, physical vapor deposition, and chemical vapor deposition. The crystalline alpha-Si3N4:C phase can incorporate about 6 at.% C and yields enhancement of hardness and wear resistance. Other crystalline phases contain more carbon, for example, Si2CN4.