The synthesis of sp(3)-bonded solid-state carbon nitrides such as beta-C3N4, a potentially technologically important hard material, remains a challenge. The stability, characterization, and synthesis of both sp(2)- and sp(3)-bonded carbon nitrides is reviewed. A discussion of the alternative structures that have been proposed for C3N4 based on local density approximation (LDA) calculations is included. Recommendations are also made as to the directions that should be pursued for success in this area of research.