For lead halide perovskite solar cells to be considered for large‐scale commercial applications, the active material must be proven to be fundamentally stable under relevant operating conditions, such as exposure to light, heat, ambient environment, and electrical bias. Reversible and irreversible effects upon applying an electric field under different environmental conditions are identified. The application of an electric field in inert conditions leads only to a reversible poling on a time scale of minutes, whose distribution is mapped throughout the semiconductor film. It is also found that the presence of moisture, and in general of small polar and hydrogen‐bonding molecules, results in an irreversible degradation in the presence of the electric field, which happens in a time scale of hours under conditions relevant for photovoltaic operation. The measurements here suggest that the irreversible field‐induced degradation in air occurs via a hydrated phase, in which the organic cation is loosely bound and can drift in response to an electric field, finally degrading the material to PbI₂. This has direct relevance to perovskite solar cells; hysteretic behavior in current–voltage curves is aggravated by the presence of moisture while devices aged under load accelerates degradation.