<p>Metal additive manufacturing (AM) has emerged as a transformative production technology, enabling the fabrication of geometrically complex, high-performance metallic components across aerospace, biomedical, automotive, and construction sectors. However, as-built metal AM parts consistently exhibit defects including surface roughness, porosity, anisotropic microstructures, residual stresses, and dimensional inaccuracies that limit their functional performance and industrial adoption. Post-processing has therefore become an indispensable step in bridging the gap between as-built part characteristics and end-use performance requirements. This paper presents a comprehensive review of post-processing techniques for additively manufactured metallic parts. The review begins with an overview of principal metal AM processes and their associated defect mechanisms. A detailed discussion of available post-processing techniques is then presented, organized by their underlying physical mechanisms: mechanical and abrasive-based, thermal and energy-beam-based, chemical and electrochemical, non-conventional machining-based, bulk densification, and emerging methods including hydrodynamic cavitation abrasive finishing. The roles of evolving standards, artificial intelligence, and hybrid additive-subtractive platforms in advancing post-processing are also examined. The current state of the field, technique-specific and systemic challenges, and future research directions are discussed, with emphasis on design-for-post-processing workflows, physics-based modeling, automation, standardization, and multi-material post-processing. Comparative tables summarizing technique capabilities, limitations, and research gaps are provided to guide researchers and practitioners.</p>