This book discusses the dynamical and rheological properties of high molecular weight entangled polymer melts, commonly known as viscoelastic subdiffusive fluids. Unlike dilute liquids, these materials exhibit molecular transport within a sublinear regime, meaning the timescale of diffusive transport is sublinear. The text emphasizes the potential of fractional calculus in modeling these fluids and introduces a novel fractional model to investigate regions of spatiotemporal instability in channel flows. At the microscale, the entanglement of polymer chains leads to localized, non-homogeneous regions with increased viscosity, which manifest as spatiotemporal macrostructures at the macroscale. To capture these macrostructures within the flow, direct numerical simulations are employed using a newly developed, physically realizable structure tensor, contributing to a deeper understanding of this complex class of fluids.