The switching mechanisms of atomic switches based on poly (ethylene oxide) (PEO) are systematically investigated. By using self-assembled PEO and Ag-PEO thin films, stack-structured devices exhibit stable bipolar switching behavior over 10(3) cycles. Direct observation of fi lament growth behavior in planar-structured devices reveals the effects of the polymer thin-film morphology, and the presence of electrochemically active electrodes, on the switching characteristics.