This study investigates bipolar and nonpolar resistive-switching of HfO₂ with various metal electrodes. Supported by convincing physical and electrical evidence, it is our contention that the composition of conducting filaments in HfO₂ strongly depends upon the metal electrodes. Nonpolar resistive-switching with the Ni electrode is attributed to the migration of metal cations and the corresponding electrochemical metallization. Conversely, oxygen-deficient filaments induced by anion migration are responsible for bipolar resistive-switching. It was also found that the characteristic nature of the conducting filaments influences many aspects of switching characteristics, including the switching power, cycling variations, and retention at elevated temperatures.