Plastics are ubiquitous in our modern world. It difficult to find a manufactured object that does not contain at least some sort polymeric (plastic) components. This ubiquity, in large part, reflects the ease with which polymers can be formed at low temperatures and pressures into arbitrary shapes through processes that induce flow of the viscous polymer melt into the cavity of a mold or die. These molding processes can induce severe visco-plastic deformation of the melt (beyond simple Newtonian flow) and have the potential to generate internal or residual stresses that, over extended periods of time, can lead to physical distortions of the component. Over the past 60 years, the polymer processing community has worked hard to develop the constitutive equations to quantify the rheological response of these viscous polymer melts under large scale deformations to establish the paradigms to optimize these forming processes and produce well-controlled, high-quality, and robust polymeric parts.