Standards define common content and performance expectations for all students in particular grades or age groups. They are derived from analysis of the structure of the core school disciplines and from efforts to reach consensus about societal goals. The content expectations tend to be aspirational, and the expected performance levels tend to be a negotiated balance between the desire to be rigorous and challenging and the need to be realistic in terms of likely failure rates on the assessments used to measure performance. In contrast, learning progressions represent hypotheses about how students’ understanding actually develops given particular instructional experiences, and they can be tested and validated against further empirical observations of the order and rate in which students’ understanding and skill do in fact develop given similar instruction. They also can be modified by evidence on what happens when instruction varies. Instead of making assumptions about what should happen, they focus on what does happen, given variation among students and their instructional opportunities.