Due to their small size and flight regime, coupling of aerodynamics, structural dynamics, and flight dynamics is critical for Micro Aerial Vehicles. This paper presents a computational framework for simulating structural models of varied fidelity and a Navier-Stokes solver, aimed at simulating flapping and flexible wings. The structural model utilizes either (i) the in-house developed UM/NLABS, which decomposes the equations of 3-D elasticity into cross-sectional and spanwise analyses for slender wings; or (ii) MSC.Marc, which is a commercial finite element solver capable of modeling geometrically-nonlinear structures of arbitrary geometry. The flow solver employs a well tested pressure-based algorithm implemented in STREAM. A NACA0012 cross-section rectangular wing of aspect ratio 3, chord Reynolds number of 3x10 4 , and reduced frequency varying from 0.4 to 1.82 is investigated. Both rigid and flexible wing results are presented and good agreement between experiment and computation are shown regarding tip displacement and thrust coefficient. Issues related to coupling strategies, fluid physics associated with rigid and flexible wings, and implications of fluid density on aerodynamic loading are also explored in this paper.