The rigorous analysis of the Euler and Navier-Stokes equations (and other models of fluid dynamics) can contribute to our understanding of turbulence. Results on regularity often exclude physical singularities of particular types, and so can influence coarser-scale modelling. Progress on questions that appear at first purely mathematical, such as the Onsager Conjecture (minimal smoothness for conservation of energy in the Euler equations), can provide insights towards foundations of the classical theory of turbulence, namely the dissipation anomaly (finite energy dissipation in the limit of zero viscosity). Analysis of boundary layers is central to many asymptotic approaches to fluid flows, and key mathematical questions remain here too, closely related to convergence of Navier-Stokes to Euler flows.