Morphological development during crystallization of thin films of poly(ethylene oxide) (PEO) / poly(methyl methacrylate) (PMMA) blends has been reported. Studies have focused on the effects of the blend composition, PMMA molecular weight, film thickness, and crystallization temperature on the observed crystal morphology. As the blend composition was varied from 90 to 30 wt% PEO, the crystal morphology varied from spherulites to needles and dendrites. Variation of the crystallization temperature and PMMA molecular weight resulted in similar changes in morphology. A morphological map demonstrating the roles of the experimental controls on the observed crystal morphology has been developed. This map was used as a tool for more detailed studies of the observed morphologies and morphological transitions. The dendritic region of the map (~ 30 = 40 wt% PEO) was studied in detail. Changes in the diffusion length were achieved through variation of the PMMA molecular weight, and were shown to influence the secondary sidebranch spacing. Sidebranch spacing measurements revealed that coarsening of the dendritic microstructure occurred well after the competition between diffusion fields of neighboring dendrite arms vanished, indicating the existence of another coarsening mechanism. These studies of dendritic sidebranching indicate that polymer dendrites develop by mechanisms similar to those in small molecules and metals. A number of in-situ observations of morphological transitions have also been reported, including a dense-branched morphology (DBM)/dendrite transition, a DBM/stacked-needle/needle transition, and a transition from dendrites with 90o sidebranching to dendrites with 45o branching or a dense-branched morphology, both of which grow at 45o to the original dendrite trunk. The DBM/dendrite transition occurred over a range of crystallization temperatures, indicating that the transition is not sharp. Crystal growth rate measurements verified this result. The DBM/stacked-needle/needle transitions demonstrated distinct jumps in the crystal growth rate, indicating a change in the growth mechanism or direction. For the transition involving a change in the growth direction, the effective level of noise (fluctuation) was found to be important in morphological selection. The results of this work have helped to define new directions for the study of crystal morphologies, especially in the areas of spherulite formation and dendritic growth. / Ph. D.