The field of turbomachinery is undergoing major advances in aeroelasticity, and this chapter provides an overview of these new developments in the key enabling methodology of unsteady aerodynamic modeling. Also see the earlier discussions in Chaps. 8 and 9.In this chapter, we review the state of the field of computational unsteady aerodynamics, particularly frequency domain methods used for the calculation of the unsteady aerodynamic forces arising in turbomachinery aeroelasticity problems. While the emphasis here is on turbomachinery aerodynamics, the methods described in this chapter have analogues for the analysis of isolated airfoils, wings, and even whole aircraft, as well as rotorcraft.The two main aeromechanics problems in turbomachinery are flutter and gust response. In the flutter problem, the unsteady aerodynamic loads acting on a cascade of turbomachinery airfoils arise from the motion of the airfoils themselves. In the gust response problem, the original excitation arises away from the blade row in question. Typical sources of excitation are wakes or potential fields from neighboring blade rows or support structures (struts), inlet distortions, and hot streaks from the combustor.More recently, a third class of aeromechanical problems has been identified [1]. This class of problems is akin to galloping of power lines or buffeting of aircraft wings. Examples include so-called separated flow vibrations and non-synchronous vibrations. In separated flow vibrations, the flow over a row of airfoils is separated, or nearly so. The flow itself is unstable, producing unsteady air loads with broadband frequency distributions that excite the airfoil at all frequencies producing a large response at the natural structural frequencies of the airfoil. Non-synchronous flow vibrations, on the other hand, are similar to separated flow vibrations in that the source of the excitation is thought to be a fluid dynamics instability (rather than an aeroelastic instability), except that non-synchronous vibrations can also occur well away from a stalled condition and the response tends to be at a single frequency. If this fluid dynamic natural frequency happens to be close to a structural natural frequency, then the fluid dynamic instability frequency can "lock on" to the structural frequency of the airfoils resulting in a large-amplitude vibration.