Ultrasonic metal welding is a solid-state bonding process in which ultrasonic vibrations create friction-like motion between two surfaces, causing deformation and shearing of asperities to bring metal-to-metal contact and bonding. Applications include the electrical/electronic, automotive, aerospace, and medical products industries. While most metals and many dissimilar combinations can be ultrasonically welded, the widest uses involve the softer alloys of copper, aluminum, and nickel. The chapter covers the principles of the process, types of welding equipment, and the mechanics and metallurgy of the ultrasonic weld. Several applications are reviewed, as are process advantages and disadvantages with notes on future trends.