This study investigates the pore evolution mechanisms during directed energy deposition (DED) additive manufacturing using in situ X-ray imaging and multi-physics modeling. The research quantifies five key mechanisms contributing to pore formation, migration, pushing, growth, removal, and entrapment: (i) bubbles from gas-atomized powder enter the melt pool and migrate circularly or laterally; (ii) small bubbles can escape from the pool surface, coalesce into larger bubbles, or be entrapped by solidification fronts; (iii) larger coalesced bubbles remain in the pool for extended periods, pushed by the solid/liquid interface; (iv) Marangoni surface shear flow overcomes buoyancy, keeping larger bubbles from popping out; and (v) once large bubbles reach critical sizes, they escape from the pool surface or are trapped in DED tracks. These mechanisms provide insights into pore minimization strategies for DED. The study also highlights the importance of industrial-relevant conditions and high temporal resolution in understanding pore dynamics, which are crucial for developing high-performance DED components with minimal porosity.This study investigates the pore evolution mechanisms during directed energy deposition (DED) additive manufacturing using in situ X-ray imaging and multi-physics modeling. The research quantifies five key mechanisms contributing to pore formation, migration, pushing, growth, removal, and entrapment: (i) bubbles from gas-atomized powder enter the melt pool and migrate circularly or laterally; (ii) small bubbles can escape from the pool surface, coalesce into larger bubbles, or be entrapped by solidification fronts; (iii) larger coalesced bubbles remain in the pool for extended periods, pushed by the solid/liquid interface; (iv) Marangoni surface shear flow overcomes buoyancy, keeping larger bubbles from popping out; and (v) once large bubbles reach critical sizes, they escape from the pool surface or are trapped in DED tracks. These mechanisms provide insights into pore minimization strategies for DED. The study also highlights the importance of industrial-relevant conditions and high temporal resolution in understanding pore dynamics, which are crucial for developing high-performance DED components with minimal porosity.