In the online world, user engagement refers to the quality of the user experience that emphasizes the phenomena associated with wanting to use a web application longer and frequently. User engagement is a multifaceted, complex phenomenon, giving rise to a number of approaches for its measurement : self-reporting (e.g., questionnaires) ; observational methods (e.g., facial expression analysis, desktop actions) ; and web analytics using online behavior metrics. These methods represent various trade-offs between the scale of the data analyzed and the depth of understanding. For instance, surveys are hardly scalable but offer rich, qualitative insights, whereas click data can be collected on a large-scale but are more difficult to analyze. Still, the core research questions each type of measurement is able to answer are unclear. This talk will present various efforts aiming at combining approaches to measure engagement and seeking to provide insights into what questions to ask when measuring engagement.