Additive manufacturing (AM) of metals offers unprecedented design flexibility and mechanical performance, making it a promising technology for high-performance and safety-critical applications. However, its widespread adoption remains limited by several persistent challenges. AM processes inherently generate complex, process-induced microstructures that result in significant heterogeneity, leading to unpredictable mechanical behavior. These variations stem from differences in solidification modes, thermal gradients, and cooling rates across processes such as laser powder bed fusion (L-PBF), laser directed energy deposition (L-DED), and wire-arc additive manufacturing (WAAM). Additionally, mechanical characterization is often time-consuming, and developing new AM materials typically requires extensive experimentation to achieve full density and reliable properties. These limitations, compounded by slow deposition rates, impede rapid innovation and deployment of AM technologies.This dissertation addresses these barriers through three key objectives: (1) to uncover and quantify process-specific heterogeneity across multiple length scales, (2) to establish rapid and reliable mechanical property evaluation methods tailored for AM metals, and (3) to investigate their mechanical response under a wide range of conditions, including varied temperatures, strain rates, and stress states. The work focuses on AM 316L stainless steel (SS), systematically examining its microstructure–property relationships as produced by L-PBF, L-DED, and WAAM. The first part explores location-dependent mechanical properties (LDMP) caused by the layer-by-layer thermal history of AM. Microstructural analyses reveal pronounced differences between the top and bottom layers of L-PBF builds in terms of grain morphology, elemental segregation, dislocation density, and residual stress. These variations correlate with increased strength and strain rate sensitivity in the bottom layers. A mechanistic model is developed to generalize LDMP behavior across AM processes, identifying cooling rate as the dominant factor controlling microstructural and mechanical heterogeneity. The second part examines the hardness–strength relationship in AM metals. While the traditional Tabor relationship assumes a constant proportionality factor (C = 3) between hardness and strength, this work demonstrates its inconsistency in AM materials due to microstructural features such as high work hardening, residual stress, and common defects. A more reliable correlation is found between hardness and the flow stress at 8% strain (σ₀.₀₈), which yields a consistent HV/σ₀.₀₈ ratio near 3 across processes. These findings enable more efficient and predictive mechanical assessment of AM metals. In the third part, the initiation of microplasticity and corresponding activation volumes in AM 316L SS and copper are investigated via nanoindentation and stress relaxation. L-PBF materials exhibit significantly higher resistance to microplasticity than their annealed, coarse-grained counterparts, requiring greater stress to activate dislocation motion. Nanoindentation-derived activation volumes (~1 b³) suggest dislocation nucleation dominates at small scales, while stress relaxation tests indicate larger-scale plasticity mechanisms (10–100 b³). These results highlight the influence of AM-specific microstructures on early-stage deformation. The fourth part focuses on back stress evolution and its relation to strain hardening in AM 316L SS. L-PBF samples show the highest back stress, followed by L-DED and WAAM, all exceeding levels found in conventionally processed alloys. This trend reflects the role of microstructural heterogeneity in enhancing kinematic hardening. Additionally, a plateau in back stress is observed at a specific strain, possibly associated with twinning or cell structure evolution. Ongoing modeling efforts aim to elucidate the underlying mechanisms. The final part explores the temperature- and strain rate-dependent behavior of AM 316L SS. Across cryogenic to elevated temperatures, L-PBF samples consistently outperform L-DED in strength, ductility, and thermal stability, underscoring their suitability for demanding applications such as aerospace. Comparative evaluation of testing methods—including tensile tests, nanoindentation, strain rate jump, and stress relaxation—provides practical guidance for selecting suitable characterization techniques tailored to AM materials. Altogether, this dissertation establishes a unified framework for understanding and predicting the mechanical behavior of AM metals. By linking microstructure to mechanical performance and advancing both modeling and testing approaches, this work supports faster characterization, process optimization, and broader implementation of AM in structural applications.