Aerospace technology has established the demand for thermally stable polymers which retain useful properties at high temperatures. The reliability of a re-entry shield is directly dependent on the performance of the polymers used as ablators. Polymeric ablation materials are sacrificial and are classified as subliming and charring ablators. Subliming polymeric ablators function by (a) depolymerization-vaporization, (b) pyrolysis-evaporation, and (c) melting-vaporization. Polymers which depolymerize quantitatively and reversibly are ideal subliming ablators. Under hyperthermal conditions, charring polymeric ablators must generate a large amount of char residue which will act as a binder at high temperatures for the reinforcements embedded in the char layer. Generally, the higher the thermal stability of the polymer, the higher the yield of char. The maximum thermal stability in polymers is usually obtained when: (a) thermally unreactive ring structures constitute a major portion of the polymer, (b) maximum use is made of resonance stabilization, (c) high bond energies exist between the atoms in the polymer, and (d) the cohesive energy density between polymer chains is high. Carbocyclic polymers, including the graphites, have high thermal stabilities but offer difficulties of fabrication. Thermally stable heteroaromatics which pass through tractable prepolymer stages are more readily prepared and fabricated; their char-forming properties in relation to structure are discussed. The values of “true” and “practical” thermal stabilities as applied to ablators are also considered. The need for a multidiscipline “oecumenical” approach to solving difficult problems is stressed. Some recent developments in the methods of syntheses of thermally stable polymers, including isotropic graphitic structures, are surveyed.