<p>Diffusion bonding (DB) represents a pivotal solid-state joining technology where materials coalesce with minimal macroscopic deformation, primarily driven by atomic diffusion at the interface under controlled temperature and pressure. This extensive review systematically synthesized the fundamental principles and recent advancements in DB, encompassing its two main variants: conventional solid-state DB and transient liquid phase (TLP) diffusion bonding. The manuscript begins by detailing the underlying mechanisms of DB, focusing on diffusion-controlled mass transport governed by Fick’s laws, the kinetics of layer growth described by an Arrhenius-type relationship, and the critical role of void closure mechanisms, which integrate plastic/creep deformation and diffusion processes. A central theme of the review is the analysis of key process parameters that govern joint quality. The significant influences of surface roughness and inherent surface oxides are thoroughly examined, as they dictate the initial contact area, void elimination, and the formation of brittle barrier layers. The strategic use of interlayers is highlighted as a primary method to overcome the challenges of bonding dissimilar materials, which often have mismatched physical/chemical properties. The review concludes that while DB and TLP bonding are established commercial processes, future work should focus on parameter optimization for economic and ecological benefits, minimizing reliance on expensive interlayers and complex surface treatments and further elucidating the microstructural evolution at bonded interfaces to predict and enhance mechanical performance reliably.</p>