Organic-inorganic metal halide perovskites have attracted significant attention for low‐cost, high‐efficiency, color‐pure light‐emitting applications. However, as seen in many reports so‐far, perovskite light‐emitting diodes (PeLED) suffer from poor operational lifetime, limiting their practical use. The underlying degradation mechanism is a topic of crucial importance. Here, the degradation mechanisms of methylammonium lead bromide based PeLED are investigated. When the PeLED is electrically biased, there is an initial raise in the luminance followed by a rapid reduction in luminance and current density. Microscopic studies reveal the formation of micrometer‐sized spots that are photoluminescent but not electroluminescent. It is demonstrated that this degradation is due to the formation of gaseous compounds that leads to local delamination of the cathode, thereby reducing the electroluminescence. When the degraded cathode is substituted with a fresh cathode, the initial luminance is largely recovered. By further analyzing the buried interface of the cathode, the formation of lead bromide and gaseous methylamine due to the degradation of the perovskite layer are revealed. These insights will help to further improve the lifetime of PeLEDs. As an example, it is shown that substituting methylammonium cations by cesium leads to longer lifetimes.