Air entrainment is a key feature of many flows in the environment and engineered systems, which leads to changes in flow properties and must therefore be considered in the design process of water infrastructure. Over the past few years, new analysis techniques and creative validation technologies for air-water flow instrumentation and innovations in the application of non-intrusive and remote sensing measurement technologies have improved observations of air-water flows in laboratory and field. Considering these recent advances, this Special Collection aims to stimulate the scientific exchange and discussion on matters related to air-water flow observations that can improve fundamental understanding and design applications.