In this thesis, we discuss the development of high fidelity multiscale methods to understand fluid flow past solid interfaces. Because of the dominant surface effects at the nanoscale, the classical field based continuum models break down. Particle-based methods offer accurate insights into the flow physics but are computationally expensive. Also, the ratio of pertinent to total information (particle trajectories) from these simulations is minimal. This research entails bridging these two descriptions of the flow physics at different scales to a unified, field-based quasi-continuum framework that can provide atomic level accuracy with continuum level efficiency. First, we discuss the construction of the transport model where fluid density and transport parameters such as viscosity are assumed to be varying across the confinement. These are, in turn, incorporated using Empirical Quasicontinuum Theory (EQT) and Local Average Density Model (LADM). We elucidate the failure of the "no-slip" boundary condition at the nanoscale and its estimation using the collective diffusion coefficient from Equilibrium Molecular Dynamics (EMD) calculations. Further, we reinterpret the slip phenomenon originating from liquid-solid interfacial friction. In this context, we discuss the Generalized Langevin Equation (GLE) based single particle dynamical framework that is consistent with the EMD simulations. Our adsorption based understanding of the flow physics elucidated that the slip length does not change with the slit width. Next, the methodology of multiscale dynamical "coarse-grained" (CG) framework is further refined to incorporate multi-physics to make it viable for a variety of fluid flow situations, such as Poiseuille flow of binary mixtures and nanochannel electroosmosis. The resultant suite of multiscale models significantly reduced the computational burden from tens of thousands of hours to seconds, without trading off the accuracy of the conventional transport parameters. Finally, we demonstrate the failure of local constitutive laws in fluids when strain-rate changes appreciably compared to the fluid molecular diameter, under extreme confinement. Here, a genuinely non-local constitutive relationship between the stress and strain-rate is more appropriate, and the viscosity is interpreted as a non-local kernel instead of a material property defined pointwise. The results indicate that a non-local model performs appreciably well in capturing the strain-rate sign reversals observed from Non-Equilibrium Molecular Dynamics calculations
