The development of mesophase carbon fiber provides a major incentive for both basic and practical investigations of the chemistry of the carbonaceous mesophase. Recent work on the properties and structural chemistry of the mesophase is surveyed, with emphasis on the chemical structures formed in the pyrolysis of pure compounds, interactions between molecules in the carbonization of practical pitch materials, and the preparation of mesophase pitches. Naphthenic intermediates play useful roles in improving anisotropy and solubility and in reducing viscosity. The growing knowledge of such mechanisms as hydrogenation, oxidation, dealkylation, and solvent fractionation opens increasingly precise approaches to control the molecular order, viscosity, and reactivity of mesophase pitch. An example is the use of hydrogenation to improve the fusibility and solubility of mesophase pitch, and under certain conditions, to prepare 'dormant mesophase', an isotropic pitch that becomes anisotropic under stress.