Sustainable aircraft structures are lightweight structures that enable the continuous use of resources and minimum waste through reuse, repair, remanufacturing, refurbishment and recycling following the principles of a circular economy. The development of fast and, hence, cost-effective welding techniques for lightweight polymer composite structures plays a key role in such circular aircraft structures since: (i) it promotes the use of thermoplastic composites, which offer more remanufacturing and recycling routes than thermoset composites; (ii) it enables modularity and hence ease of disassembling and reuse at the end of the life of the aircraft or during maintenance. One of the fastest and more versatile existing composite welding techniques is ultrasonic welding. This presentation explains the main principles of ultrasonic welding, it discusses its two main upscaling strategies: sequential and continuous ultrasonic welding, their current level of development and potential applications, and, finally, their remaining challenges.