Lead halide perovskites are the new rising generation of semiconductor materials due to theirunique optical and electrical properties. The investigation of the interaction of halide perovskites and light is a key issue not only for understanding theirphotophysicsbut also for practical applications. Hence, tremendous efforts have been devoted to this topic andbrunch into two:(i)decompositionof the halide perovskites thin films under light illumination and(ii)influence of light soaking on their photoluminescence (PL) properties. In this review, we for the first time thoroughly compare the illumination conditions and the sample environment to correlate the PL changes and decomposition of perovskite under light illumination. Inthe case of vacuum and dry nitrogen, PL of the halide perovskite ( PbI3–xClx, PbBr3–xClx, MAPbI3) thin films decreases due to the defects induced by light illumination, and under high excitations the thin film even decomposes. In thepresence of oxygen or moisture,light induces the PL enhancement of halide perovskite (MAPbI3) thin films at low light illumination, while increasing the excitationcauses the PL to quench and perovskite thin film to decompose. In the case of mixed halide perovskite (MAPb(BrxI1-x)3) light inducesreversible segregation of Br domains and I domains.