We provide a new measure of work motivation and show that motivation shapes the effects of team incentives and observation by peers on performance. In particular, we measure motivation to work hard as the deviation from the money-maximizing benchmark in a real-effort experiment. While we find that average output increases in response to team incentives and observation, we find that highly motivated workers do not respond. The reason is that highly motivated workers already work hard and increasing effort even further is very costly to them.