How does self-efficacy affect interest? The interest-and-interests model assumes that factors that induce interest-novelty, complexity, conflict, and uncertainty-do so non-linearly. Self-efficacy should thus affect interest quadratically, because it reflects uncertainty about an activity's outcome. When self-efficacy is low, interest is low because the activity's outcome is certain. When self-efficacy is moderate, the person's success on the task seems likely, but not inevitable. But as self-efficacy becomes very high, success seems completely certain, and the task is thus uninteresting. Two experiments tested these predictions. Experiment 1 asked people to rate the interestingness of differentially difficult activities; Experiment 2 manipulated self-efficacy regarding a fuzzy dart game. In both experiments, interest was a quadratic function of self-efficacy. Implications for theories of vocational interest development and change are considered.