Abstract：The main focus of research on lightning physics is cloud to ground lightnings including its initial formation in the clouds, propagation in the air and contact with ground structures. In a large cloud area with strong electric field, the corona at the tips of the water thread formed during passing collision between two raindrops or the corona on the surface of ice like hydrometerors will trigger lightning. The propagation of lightning in air is a transformation process from corona to electric arc. The key process is the formation of an upward leader from the ground structure and its joining up with the downward leader. Research physics should focus on the discharge to ground process, together with protective measures against lightning.