Behavioral scientists have long been intrigued with the investigation of change. From a general perspective, questions of interest in such inquiry might be: “Do the rates at which children learn differ in accordance with their interest in the subject matter?” From a more specific perspective, such questions might include: “To what extent do perceptions of ability in particular school subjects change over time?” and “Does the rate at which self-perceived ability in math and/or science change differ for adolescent boys and girls?” Answers to questions of change such as these necessarily demand repeated measurements on a sample of individuals at multiple points in time. This chapter addresses these types of change-related questions.