Metal Halide Perovskite is a star material that draws tremendous attention in both research andindustry. Very high-quality semiconducting materials can be made with simple solution processesMoreover, alloying with different cations and anions in the ABX3 structure has demonstrated an excellentturnability of the materials electronic properties, The perovskite film formation relies on fast and efficiention coordination during the liquid-to-solid phase transition. Photovoltaic and light-emitting devices basedon perovskites have already achieved superb performance. However, challenges remain in realizing severalimportant high-quality perovskite compositions and preventing the materials from degradation.Thereforeunderstanding crystallization and defect formation is vital for further advancing the technology. ln thispresentation, we will share some progress of our research on the above topics. We will demonstrate twoi n-situ techniques to probe the crystallization and degradation of perovskite materials and devices. A highlysensitive quantum efficiency technique which has a wide dynamic range of 109 can detect fA photocurrentgenerated in defect states in perovskite during the device operation, An in-situ dynamic Pl. probe can reveal information on ion coordination during the spin-coating and thermal annealing.