Liquid degassing by ultrasound is a common and well-known technique. Important applications comprise liquid metal melts and ultrasonic cleaning. In the latter case, degassing often occurs as a necessary precursor of hard (“transient”) cavitation. Here we analyse the degassing process from a microscopic to mesoscopic view. This includes gas-collecting mechanisms like rectified diffusion and bubble attraction and coalescence. On this basis, estimates of overall degassing rates are attempted and the potential insight into existence and quantity of so-called “bulk nano-bubbles” via ultrasonic degassing experiments is explored.