Four crystal types (a, b, g, d) and some mesophases/sub-modiยฎcations have been identiยฎed and discussed in syndiotactic polystyrene (sPS). The a-and b-forms are the main crystal packing forms in thermally-processed sPS, while the g-and d-crystals are identiยฎed only in solvent-treated sPS. In addition, the d-and g-forms are of a monoclinic crystal cell (with helical chain conformation) and the cell dimensions depend on the types and amount of residual solvent trapped in the crystal. The d-and g-crystal in solvent-treated sPS are more like mesophases that transform readily to the a-, b 00 -or b 0 -crystal upon heating the solvent-treated sPS to high temperatures near melting. This review thus focuses on studies of the a, b-crystals in sPS, and provides comprehensive discussions on the thermal behavior, crystal structures, thermodynamics, kinetics, and stability of these two major crystal packings (a vs. b) in sPS upon melt crystallization in comparison with cold crystallization. Analyses of melting behavior, diffractograms, or IR spectra, etc. of sPS can be complicated by the presence of co-existing polymorphic crystals. In general, a total of four melting peaks (labeled as P-I, P-II, P-III, P-IV from low to high temperatures) have been identiยฎed in a melt-crystallized sPS that typically contains mixed fractions of both crystals. By reยฎning the techniques of obtaining sPS with individually isolated a-or b-crystal, recent studies have been able to correct suspected inaccuracy of some thermodynamic and kinetic measurements in earlier studies and to interpret the relative stability of the various crystals in sPS. sPS samples could be prepared such that they contained purely isolated a-or b-crystal, and the individual crystal types are used for more precise characterization of analysis. The P-I and P-III melting peaks are attributed to the b 0 -crystal while the P-II and P-IV peaks are attributed to the a 00type. In addition, kinetic and thermodynamic characterization has been thoroughly performed on individually isolated crystal types. The a-crystal of sPS has a lower melting temperature than the b-crystal, with T 0 m;a 281:78C and T 0 m;b 288:78C: The crystallization kinetics of the a-crystal is a heterogeneous nucleation with higher rates while the b-crystal is a homogeneous nucleation with lower rates. The b 0 -type is more thermodynamically stable than the a 00 -type; but the a 00 -type is kinetically more favorable. In addition, although there is literature report concerning a transformation of d or g mesophase crystals to a-or b-crystal; there is no evidence showing a solidยฑsolid transition from the a-to b-crystal or b-to a-crystal during normal thermal processes. It