Filament formation and dissolution in the system Ag(Cu)/ZrO2/Pt were observed by Liu et al. [Adv. Mater. 2012, 24, 1844]. Their explanation of the phenomena is shown here to be inappropriate. Various situations, including the bipolar electrode shown in the figure, are considered and the difference between the behavior in electrochemical metallization memories (ECMs) and valence change memories (VCMs) outlined. Of crucial importance for distinguishing ECM from VCM behavior, that is, the effects of cation and anion migration, is the choice of the solid materials used to transport metal cations. A possible explanation of the phenomena is proposed.