Ultrasound has been used for motion compensation in radiotherapy since the late 1990s. It does not utilize ionizing radiation and can provide real-time imaging at a low cost. Early systems relied on freehand 2D imaging and facilitated soft-tissue image guidance for the first time. However, they suffered from strong user dependency and limited positioning accuracy, slowing their widespread utilization. Many of their drawbacks were overcome when 3D ultrasound was introduced. Innovations in hard- and software have led to the latest generation of ultrasound systems which can provide high volumetric framerates with superior image quality by using matrix array transducers. Due to the large field-of-view and the high spatiotemporal resolution, these new systems are ideally suited for real-time motion compensation tasks. By accessing the volume data directly, it is possible to interface with dynamic MLC tracking to follow tumor motion with low latency and good dosimetric results. To aid unexperienced users with the setup, tools were developed to calculate the ideal probe position and guide the user through the anatomy with real-time visual feedback or the use of augmented reality applications. Furthermore, robotic solutions for probe positioning and stabilization are currently being investigated in order to eliminate user dependency, one of the biggest challenges in ultrasound imaging. Recent developments in technology have facilitated such new approaches and helped overcome some of the previous drawbacks. The evident benefits of ultrasound imaging make it a promising modality for real-time motion compensation in radiotherapy of soft tissues.