<p>Deep-hole drilling with twist drills remains widely used for producing holes with medium to high depth-to-diameter ratios because of its high versatility, low cost, and adaptability to conventional machine tools. However, due to the confined in-hole space, the high slenderness of the drill body, and the restricted passages for chip and fluid transport, the process is prone to trajectory instability, poor chip evacuation, heat accumulation, and accelerated tool wear, which ultimately impair hole quality and reduce tool life. To address these interconnected issues, this review establishes a unified analytical framework centered on trajectory stability, chip transport, thermal management, and wear evolution, based on three coupled subsystems: cutting generation, chip evacuation and transport, and margin-contact friction. By revisiting representative studies published before 2020 to clarify the fundamental mechanisms, the review further focuses on research progress reported from 2021 to 2025, while selectively incorporating a few very recent studies for contextual updating. Particular attention is given to the mechanisms and trade-offs of geometric and margin design, the coordination of chip breaking, chip evacuation, and cooling, surface engineering and margin-contact control, as well as strategies for improving wear resistance, tool life, and bore-wall integrity. The review also discusses recent advances in integrating thermo-mechanically coupled finite element analysis, computational fluid dynamics (CFD), and particle- or multiphase-coupled simulations with experimental validation. Overall, this review provides a structured reference for understanding the machining mechanisms of deep-hole twist drilling and for the coordinated optimization of tool design and process parameters.</p>