This paper presents an exhibit assessment tool for science centers that is based on the premise, supported by learning theories and research, that the level to which a visitor is engaged by an exhibit is a direct indicator of the learning taking place. Observable behaviors are used to distinguish three stages or levels of visitor engagement described as initiation, transition, and breakthrough. A distinctive visitor engagement profile can be constructed for an exhibit that can then be used in assessing the effectiveness of subsequent changes made to the exhibit experience. It is suggested that the visitor engagement and exhibit assessment model describes and predicts relationships between exhibits, visitors, and observable learning behaviors in science centers.