Employee engagement has become one of the most popular topics in management. In less than 10 years, there have been dozens of studies published on employee engagement as well as several meta‐analyses. However, there continue to be concerns about the meaning, measurement, and theory of employee engagement. In this article, we review these concerns as well as research in an attempt to determine what we have learned about employee engagement. We then offer a theory of employee engagement that reconciles and integrates Kahn's (1990) theory of engagement and the Job Demands–Resources (JD‐R) model (Bakker & Demerouti, 2007). We conclude that there continues to be a lack of consensus on the meaning of employee engagement as well as concerns about the validity of the most popular measure of employee engagement. Furthermore, it is difficult to make causal conclusions about the antecedents and consequences of employee engagement due to a number of research limitations. Thus, there remain many unanswered questions and much more to do if we are to develop a science and theory of employee engagement.