I will present a seamless multiscale method for the study of fluid systems with unknown constitutive relations and/or unknown boundary conditions. The multiscale method aims at capturing the macroscale behavior of the fluid system; this is done by combining the conservation laws for the mass and momentum (the macro model) and the molecular dynamics (the micro model). The macro model provides the necessary constraint for the micro model and the micro model supplies the missing data (the stress or the boundary conditions) needed in the macro model. In the multiscale method, the macro and micro models evolve simultaneously using different time steps, and they exchange data at every step. The micro model uses its own appropriate (micro) time step. The macro model uses a macro time step but runs at a slower pace than required by accuracy and stability considerations in order for the micro model to relax. One of the advantages of this method is that it does not require reinitialization of the micro model at each macro time step or each macro iteration step. In this talk, I will discuss the algorithm of the multiscale method, the error analysis, and its application to complex fluids.