The new modularization framework of FAST v.8 (Jonkman 2013) was accompanied by a complete overhaul of the aerodynamics routines. AeroDyn is an aerodynamics module comprised of four submodels: rotor wake/induction, blade airfoil aerodynamics, tower influence on the blade nodes, and tower drag. Throughout the software overhaul, several improvements to the original theoretical treatments were achieved, including more accurate skewed-wake and unsteady airfoil aerodynamics, and the possibility of modeling highly flexible and curved blades. Under asymmetric conditions, such as wind shear, yawed, and tilted flow, the individual blade elements undergo variations in angle of attack that lead to unsteady aerodynamics phenomena, which can no longer be captured through the static airfoil lift and drag look-up tables. This document covers the main theory and the organization of the modularization framework of the new unsteady aerodynamics submodule (UAM)1, which includes unsteady aerodynamics under attached flow conditions and dynamic stall. The UAM can be called by either blade element momentum theory (only interface discussed in this report), or other future wake models (e.g., dynamic blade element momentum theory, or generalized dynamic wake).