We present theory suggesting that experiences at work that meet employees’ expectations of need fulfillment drive work engagement. Employees have needs (e.g., a desire to be authentic) and they also have expectations for how their job or their organization will fulfill them. We argue that experiences at work that confirm employees’ need fulfillment expectations yield a positive emotional state that is energizing, and that this energy is manifested in employees’ behaviors at work. Our theorizing draws on a review of the work engagement literature, in which we identify three core characteristics of work engagement: (a) a positive emotional state that (b) yields a feeling of energy and (c) leads to positive work-oriented behaviors. These key themes provide the foundation for further theorizing suggesting that interactions at work confirm or disconfirm employees’ need fulfillment expectations, leading to different levels of engagement. We extend our theorizing to argue that confirmation, or disconfirmation, of different need expectations will yield emotional experience of varying magnitudes, with confirmation of approach-oriented need expectations exerting stronger effects than the confirmation of avoidance-oriented need expectations. We close with a review suggesting that organizational contextual features influence the expression of these needs, sustaining or undermining the positive emotional experiences that fuel work engagement.