This study investigates pore evolution mechanisms during directed energy deposition (DED) additive manufacturing using in situ X-ray imaging and multi-physics modelling. Pores in DED components degrade mechanical performance, limiting safety-critical applications. The research identifies five mechanisms contributing to pore formation, migration, pushing, growth, removal, and entrapment: (1) gas bubbles from gas atomised powder enter the melt pool and migrate; (2) small bubbles escape or coalesce, or are entrapped by solidification fronts; (3) larger bubbles are pushed by the solid/liquid interface; (4) Marangoni surface shear flow prevents bubble popping; and (5) large bubbles escape or are trapped in DED tracks. These mechanisms guide pore minimisation strategies. DED is a promising layer-by-layer additive manufacturing technology for complex geometries. However, porosity during DED limits industrial applications due to its detrimental effect on mechanical performance. Porosity in DED components is mainly gas porosity and lack of fusion features. Gas porosity originates from feedstock, shielding gas entrapment, and gas evolution. Lack of fusion porosity results from insufficient energy input. Porosity is typically investigated with ex situ techniques, but these fail to capture pore formation and dynamics. In situ observations are needed to understand pore evolution. In situ X-ray imaging studies have been conducted on solidification dynamics, but few on DED. Pore formation during laser powder bed fusion (LPBF) was studied using in situ synchrotron X-ray imaging and multi-physics modelling, revealing high thermocapillary force can eliminate pores. Pores were also found to form at the end of the scan vector during laser turning due to keyhole depression collapse. Two studies systematically investigated pore formation during LPBF using high-speed X-ray imaging, finding pore formation can be caused by a critical instability at the keyhole bottom. However, this mechanism does not apply to DED, which has a larger laser spot size and lower energy density than LPBF. DED is normally in conduction mode with no keyhole, has a much larger melt pool, and includes powder bombardment, which can contribute to different bubble evolution and melt pool dynamics. This study uses in situ high-speed synchrotron X-ray imaging (>20 kHz) to investigate pore evolution mechanisms during DED-AM. Pore behaviour including formation, coalescence, pushing, migration, escape, and entrapment is quantified. A multi-physics and high-fidelity model is applied to validate hypothesised mechanisms. The study reveals five stages of bubble behaviour: (1) pore formation; (2) bubble coalescence and growth; (3) solid/liquid interface pushing of large bubbles; (4) large bubble entrainment in the molten pool; and (5) bubble escape or entrapment. These stages depict the life cycle of bubbles in DED AM, which repeat periodically during the building processThis study investigates pore evolution mechanisms during directed energy deposition (DED) additive manufacturing using in situ X-ray imaging and multi-physics modelling. Pores in DED components degrade mechanical performance, limiting safety-critical applications. The research identifies five mechanisms contributing to pore formation, migration, pushing, growth, removal, and entrapment: (1) gas bubbles from gas atomised powder enter the melt pool and migrate; (2) small bubbles escape or coalesce, or are entrapped by solidification fronts; (3) larger bubbles are pushed by the solid/liquid interface; (4) Marangoni surface shear flow prevents bubble popping; and (5) large bubbles escape or are trapped in DED tracks. These mechanisms guide pore minimisation strategies. DED is a promising layer-by-layer additive manufacturing technology for complex geometries. However, porosity during DED limits industrial applications due to its detrimental effect on mechanical performance. Porosity in DED components is mainly gas porosity and lack of fusion features. Gas porosity originates from feedstock, shielding gas entrapment, and gas evolution. Lack of fusion porosity results from insufficient energy input. Porosity is typically investigated with ex situ techniques, but these fail to capture pore formation and dynamics. In situ observations are needed to understand pore evolution. In situ X-ray imaging studies have been conducted on solidification dynamics, but few on DED. Pore formation during laser powder bed fusion (LPBF) was studied using in situ synchrotron X-ray imaging and multi-physics modelling, revealing high thermocapillary force can eliminate pores. Pores were also found to form at the end of the scan vector during laser turning due to keyhole depression collapse. Two studies systematically investigated pore formation during LPBF using high-speed X-ray imaging, finding pore formation can be caused by a critical instability at the keyhole bottom. However, this mechanism does not apply to DED, which has a larger laser spot size and lower energy density than LPBF. DED is normally in conduction mode with no keyhole, has a much larger melt pool, and includes powder bombardment, which can contribute to different bubble evolution and melt pool dynamics. This study uses in situ high-speed synchrotron X-ray imaging (>20 kHz) to investigate pore evolution mechanisms during DED-AM. Pore behaviour including formation, coalescence, pushing, migration, escape, and entrapment is quantified. A multi-physics and high-fidelity model is applied to validate hypothesised mechanisms. The study reveals five stages of bubble behaviour: (1) pore formation; (2) bubble coalescence and growth; (3) solid/liquid interface pushing of large bubbles; (4) large bubble entrainment in the molten pool; and (5) bubble escape or entrapment. These stages depict the life cycle of bubbles in DED AM, which repeat periodically during the building process