A general technique for constructing reduced order models of unsteady aerodynamic flows about two-dimensional isolated airfoils, cascades of airfoils, and three-dimensional wings was developed. The technique was applied to an unsteady incompressible vortex lattice model. The eigenmodes of the system, which may be thought of as aerodynamic states, were computed and subsequently used to construct computationally efficient, reduced order models of the unsteady flow field. It was also shown how the reduced order model may be incorporated into an aeroelastic flutter model.