One of the biggest challenges in manufacturing automotive lithium-ion batteries is to achieve consistent weld quality in joining multiple layers of dissimilar materials. Although most fusion welding processes face difficulties in such joining, ultrasonic welding overcomes those difficulties due to its solid-state process characteristics. However, inconsistency of weld quality still exists because of limited knowledge on the weld formation through the multiple interfaces. This chapter aims to establish real-time phenomenological observation on the multilayer ultrasonic welding process by analyzing the vibration behavior of metal layers. Such behavior is characterized by a direct measurement of the lateral displacement of each metal...