Polymer melts display complex flow behavior governed by molecular architecture, entanglement density, and nonlinear deformation history. Our research addresses fundamental and applied questions in the rheology of entangled linear polymers, ring polymers, polymer blends, and functionalized melts under shear and extensional flow. We investigate how molecular dynamics under flow determine macroscopic processing behavior, with particular emphasis on nonlinear rheology, flow instabilities, crystallization, and structure development during processing.