This thesis is made of two parts. In the first part, we study the rheology and nonequilibrium structure of fluids using the nonequilibrium ensemble method and a computational framework is laid toward a theory of steady state. The grand canonical partition function is computed analytically and used to study a gas mixture under shear. The dependence of the viscometric functions on the shear rate is presented and an exact constitutive relation between the kinetic-stress and the generalized potentials is obtained. The system was found to exhibit a shear thinning behavior. The Monte Carlo method in combination with generalized hydrodynamics was used to study the shear rate dependence of viscosity and the nonequilibrium structure of simple liquids. The fluid exhibited shear thinning, and a generalization of the Ree-Eyring equation was derived. This formula accurately fits the results of nonequilibrium molecular dynamics results and the relaxation time is computed from the theory. A set of self consistent integral equations is proposed for polyatomic fluids, which could be used for studying equilibrium and nonequilibrium molecular fluids. The prediction of these equations compared positively with simulation results for a number of molecular models.