Great interest exists in adding long fibers into polymeric fluids due to the increase in properties associated with the composite, as compared to the neat resin. These properties, however, are dependent on the fiber orientations generated during processing, such as injection molding. In an effort to optimize industrial processing, optimize mold design, and maximize desired properties of the final part, it is highly desirable to predict long fiber orientation as a function of processing conditions. The purpose of this research is to use rheology as a fundamental means of understanding the transient orientation behavior of concentrated long glass (> 1mm) fiber suspensions. Specifically, this research explores the method of using rheology as a means of obtaining stress tensor and orientation model parameters needed to accurately predict the transient fiber orientation of long glass fiber reinforced polypropylene, in a well-defined simple shear flow, with the hopes of extending the knowledge gained from these fundamental experiments for the use of simulating processing flows, such as injection molding. Two fiber orientation models were investigated to predict the transient orientation of the long glass fiber systems explored. One model, the Folgar-Tucker model, has been particularly useful for predicting fiber orientation in short glass fiber systems and was used in this paper to assess its performance with long glass fibers. A second orientation model, one that accounts for the semi-flexibility of fibers, was extended to describe non-dilute suspension and coupled with an augmented stress tensor that accounts for fiber bending. Stress tensor and orientation model parameters were determined (in all cases) by best fitting these coupled equations to measured stress data obtained using a sliding plate rheometer. Results showed the semi-flexible orientation model and stress tensor combination, overall, provided improved rheological results as compared to the Folgar-Tucker model when coupled with the stress tensor of Lipscomb (1988). Furthermore, it was found that both stress tensors required empirical modification to accurately fit the measured data. Both orientation models provided encouraging results when predicting the transient fiber orientation in a sliding plate rheometer, for all initial fiber orientations explored. Additionally, both orientation models provided encouraging results when the model parameters, determined from the rheological study, were used for the purpose of predicting fiber orientation in an injection molded center-gated disk. / Ph. D.