A novel approach for implementing a nonlinear unsteady frequency domain harmonic balance solution technique about existing implicit computational fluid dynamic flow solvers is presented. This approach uses an explicit discretization of the harmonic balance source term, which enables the harmonic balance method to be applied to existing implicit flow solvers with minimal need for modification to the underlying implicit flow solver code. The resulting harmonic balance solver can then be used for modeling nonlinear periodic unsteady flows. The methodology is applied to the OVERFLOW 2 flow solver code, and results are presented for transonic viscous flow past an unsteady pitching airfoil section. Unsteady aerodynamic and aeroelastic results for the F-16 fighter wing are also presented. Copyright © 2009 by Jeffrey P. Thomas, Chad H. Custer, Earl H. Dowell, and Kenneth C. Hall.