The non-Newtonian flow characteristics of polymers and other complex fluids is intimately related to the ability of flow fields to perturb molecular or meso-scale structure. Consequently, modern research emphasizes the molecular or microstructural origins of complex rheological behavior. Further, technological applications of polymers often rely on flow-induced structural changes (i.e. molecular orientation) that affect the resulting material properties. This talk will describe efforts to establish in situ x-ray scattering as a tool to probe the structure of complex polymer fluids under flow. High brilliance synchrotron sources coupled with advances in detector technology enable real-time studies of transient structural dynamics, which can, in many cases, be directly linked to macroscopic rheological behavior. The presentation will focus on recently developed capabilities to study structure developments in polymer melts subjected to well-defined uniaxial extensional flow. The instrument is based on the commercially available ‘SER’ extensional flow fixture, which has been incorporated into a home-built convection oven designed for operation in a synchrotron beam line environment, providing access for incident and scattered x-ray beams. Applications of the instrument to the study of extensional flow-induced alignment of ordered, microphase-separated block copolymer melts will be discussed, exploring the impact of degree of segregation, initial orientation state, and microphase morphology (cylindrical vs spherical microdomains).