<p>Owing to melting and solidification, mainstream fusion-based metal additive manufacturing processes are&#xa0;limited by steep thermal gradients, hot cracking, and columnar grain formation. Deformation-based additive manufacturing offers a different route, wherein metallurgical continuity is implemented through intense plastic deformation. This article examines microstructure development and design in such solid-state approaches, using additive friction stir deposition and cold spray as representative case studies. Although both rely on large deformation, they operate in distinct deformation processing regimes spanning wide ranges of temperature, strain, strain rate, and time&#xa0;scale. We present a unified perspective linking processing conditions to the mechanisms that govern dynamic microstructure and phase evolution, with a highlight of the resulting contrasting microstructural features—refined equiaxed grain structures versus highly nonequilibrium, defect-rich bimodal structures. Building on this foundation, we discuss microstructure design strategies in these deformation-based additive processes along with property optimization opportunities that are inaccessible to their fusion-based counterparts.</p> Graphical abstract <p></p>