Keywords: Turbulent energy dissipation, eddy viscosity, stability Abstract: Variational techniques have been successful in establishing limits on transport in turbulent flow systems (e.g heat in convection, mass in pressure-driven shear flow or momentum in boundary-driven flows). However, there is typically a significant discrepancy between the limit derived and observations as well as a disconnect between the theoretical `optimal flow' solution and what is actually seen. After giving examples of this, I will discuss some past work directed at closing this gap and motivate the use of plausible stability criteria.