A nonlinear aeroelastic model for flapping micro air vehicle wings undergoing prescribed rigid body motion and moderate-to-large flexible deformation is presented. The aeroelastic model is obtained by coupling a nonlinear structural dynamic model based on the MARC code with approximate unsteady aerodynamic loads. The aeroelastic response is obtained using an updated Lagrangian method. The aerodynamic model is based on potential flow that uses a combined circulation/vorticity approach to compute the unsteady aerodynamic loads. An important ingredient of the aerodynamic model is the inclusion of a leading edge separation and subsequent vortex formation. The airfoil is modeled by bound vorticity and the wake is represented by point vortices. The unsteady loads computed using this approach are used to examine the effect of wing flexibility on the response. Preliminary aeroelastic response results indicate that, for the parameters considered, the effect of aerodynamic loads is relatively minor compared to the effect of inertial loads; furthermore, wing flexibility may have a favorable effect on lift generation.