“EIT waves” are globally-propagating disturbances in the solar corona that remain controversial almost twenty years since first being observed by the Extreme ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory (SOHO). Originally identified as waves, opposing observations from a myriad of instruments and passbands have questioned this assumption and resulted in conflicting interpretations being proposed to explain this phenomenon. The launch in 2010 of the Solar Dynamics Observatory (SDO) has revolutionised our understanding of these features, providing high temporal and spatial resolution observations of “EIT waves” and their evolution through the solar corona. These observations have allowed the development of automated tracking algorithms and better data-driven simulations, which, combined with detailed case studies and statistical analysis of event databases, are leading to a better understanding of “EIT waves”. Analysis of these features is therefore moving beyond a simple investigation of their physical nature towards using them to study the corona through which they propagate. Here, we discuss the theories proposed to explain “EIT waves”, placing them in the context of the observations available at the time and showing how they have been updated with improved observations and models. We then look towards the future, examining how “EIT waves” may be used to study the structure of the solar corona and infer the properties of their associated coronal mass ejections.