In Large Eddy Simulation (LES) the filtered Navier-Stokes equations are solved instead of the full equations. An extra term appears in the filtered Navier Stokes equation when compared with the full Navier Stokes equation, which is called the "subgrid stress term". We will analyze which eddy-eddy interactions contribute to the subgrid-stress dynamics. From this analysis we will see that there is an aliasing problem in LES, which can spoil the time-forwarding of simulated fields. A proposition will be made to solve this problem in the context of a spectral LES. We will relate the filtered fields to measurements and show that the best approximation of measurable quantities is obtained only when the filtered fields are conditionally inverse-filtered. It will be shown that for all types of filter this necessary procedure of post-processing can be circumvented by the ab-initio assumption of a cut-off filter, which requires no inverse-filtering. We will demonstrate our theory on a set of field measurements in the atmospheric surface layer. With our theory the magnitude of the subgrid stress, which still has to be parameterized, is substantially reduced when compared with the classic formulation.