Additive friction stir deposition leverages the principle of friction stirring in the context of additive manufacturing to enable rapid heating and deformation of metals. Compared to other solid-state metal additive manufacturing technologies, which only involve local deformation, here the plastic deformation is global: the entire feed material undergoes severe plastic deformation at elevated temperatures. As a result, additive friction stir deposition is able to produce fully-dense as-printed material with equiaxed, fine microstructures and wrought-like mechanical properties—a unique capability distinguishing it from other metal additive processes. In this presentation, I will provide an overview of the process fundamentals underlying additive friction stir deposition, with a focus on the thermal history and plastic deformation path. Multi-channel in situ monitoring is shown to unravel the correlation between the process parameters and peak temperature, whereas tracer-based X-ray computed tomography reveals the mesoscopic shape evolution, plastic strain development, and concurrent grain structure evolution.