This chapter introduces fundamental models for unsteady aerodynamic loading, categorized into three types: analytical, numerical, and semi-empirical. Analytical models are conventionally achieved through extensions of the two-dimensional thin-airfoil steady aerodynamic theory. Numerical models are based on approximation methods to solve simplified or complete sets of fluid mechanics equations. Semi-empirical models use simplified formulations and add terms and correction functions based on experimental information on physical events associated with the movement of bodies in aerodynamic flow. In the following sections, each of the three categories is explored.