Progress and key difficulties in the synthesis and characterization of carbon nitride crystals are summarized. It could be concluded that there is no definite evidence and effective method for synthesizing a carbon nitride crystal. The main problems in the high temperature and high pressure method are the choice and preparation of the precursor materials and the effective control of thermodynamic parameters during the high pressure processing. For the plasma CVD method, the substrate atom, especially silicon, plays an important role in the synthesis of a carbon nitride crystal. The difficulties needed to be resolved using the ion beam sputtering method are how to increase nitrogen concentration in the reactants at a lower substrate temperature and to find synthesis conditions to form a high concentration of sp3C-N single bonds. Electrochemical methods can effectively reduce the activation energy and deposition temperature by using precursors containingC-N single bonds, but the problems are how to promote crystallization of the deposits and to decrease the amount of byproducts. Combined application of the above mentioned methods may be effective in the synthesis of carbon nitride crystals.