Some solid-state reactions which give rise to the formation of polymer crystals are discussed and the observation of the nascent polymer morphology is used as a guideline to learn about the reaction mechanism of solid-state polymerizations. Examples for the following three different mechanisms are treated in detail : a) crystalliration succeeding polymerization, b) simultaneous polymerization and crystallization, and c) polymerization in solid solution.The nascent morphology of poly(alky1ene)s obtained by Zicgler-Katta catalysis is taken as an example for cr!'.S/cJ//iiU/iOfl sitccwding polymerixrion. Another example is thc cationic polymerizition of 1,3,5-trioxanc from solution. In this case a thermodynamic equilibrium between thc phases of the solid crystalline polymer and the dissolved monomer is approachcd: here, by breaking up chain folds and insertion of monomeric units, an increase in thickness of thc crystals takes placc, which finally l a d s to extended chain crystals by this transacetaliation.The solid-state polymerization of I .3,5-trioxane and I ,3,5,7-tetroxanc as induced by high-energy radiation or catalysts is described as an example for sirnulrunroirs poljmerizurion and crj~s/allizarion. Possible molccular models of the chain growth are developed on the basis of thc morphological observations. Truly extended chain crystals of poly(oxymethylene), (POM), cannot be obtained generally from solid 1.3.5-trioxane or 1.3.5.7-tetroxane; the rcason for this is either chain-folding (density fluctuation along the fibre axis) or "twin"-structure formation (orientation fluctuation). The nature and origin of the "twin"-structure of POM from crystallinc 1,3.5-trioxane is discussed in detail.Topochemical polymerization of monomers with conjugated triple-bonds is an example for polJrneriiutioti in solid solittion. Thc polymer chains grow as isolated macromolecules within the monomer latticc. Since quantitative conversion can be reached in some cases without phaseseparation, thiscompriscsa method to produce macroscopic, extended chain polymer single crystals, which so far could not be prepared by anothcr method.