Metallic lead (Pb) impurities in metal-halide perovskites have attracted tremendous research concerns owing to their detrimental effects on perovskite solar cells (PSCs). However, the origins and influences of the Pb behind this issue have yet to be well understood. Herein, we show that Pb is hardly formed in the growth of halide perovskites but is easily postformed in the perovskite films with excess PbI. It is found that Pb impurities are decomposition byproducts of residual PbI in perovskites under light or X-ray irradiation. Therefore, PSCs obtained using photodegraded PbI films show large efficiency and stability losses. By contrast, the perovskite devices without detectable Pb impurities have a much better efficiency and stability. This work reveals the origins and influences of Pb in halide perovskites and provides a strategy for avoiding the formation of detrimental Pb byproducts, which would drive further enhancements in device performance of halide perovskite solar cells, detectors, etc.