<p>Directed energy deposition (DED) is a versatile metal additive manufacturing technique used for producing large components and repairing high-value parts. Despite its potential, current knowledge of DED is fragmented across process parameters, thermal history, microstructure evolution, and mechanical performance, which restricts the transferability of optimized parameter sets between different systems and materials. This review consolidates the current understanding, with a focus on laser-based DED, of how key parameters such as laser power, scanning speed, feed rate, hatch spacing, and layer height influence melt pool dynamics, thermal gradients (G), solidification rates (R), and cyclic reheating. It further examines how these thermal conditions affect solidification, phase transformations, texture development, precipitation, and defect formation in steels, titanium alloys, nickel-based superalloys, aluminum alloys, and high-entropy alloys. The relationships between microstructural features such as grain morphology and size, texture, phase constitution, and defect populations, and mechanical properties including strength, ductility, hardness, and fatigue performance are critically evaluated. Special emphasis is placed on nonlinear parameter interactions and the limitations of relying on single energy-density descriptors to predict build quality. In addition, the reviewed empirical relationships are explicitly linked to the computational modeling approaches that they inform and validate, including thermal–fluid simulations of melt pool behavior, phase-field and cellular-automaton models of solidification, and thermo-mechanical Finite Element Analysis (FEA) of residual stresses. The review concludes by highlighting unresolved challenges and proposing directions for developing transferable process maps, predictive models, and closed-loop control strategies for industrial DED applications.</p>