Measures of student growth offer a richer understanding of student performance than a moment-in-time test score alone, but measures of student growth are not created equal. In their accountability plans for implementing the Every Student Succeeds Act (ESSA), some states are using a sophisticated analysis of multiple data points that evaluate the impact of schools on student learning, while others are using simpler measures of change in student assessment results year to year. These different approaches to measurement answer different questions and tell different stories about what is happening in schools and classrooms. Leaders in all but two states have committed to measuring student growth as an indicator of academic achievement in accountability systems for elementary and middle schools, and 20 states will do so for high schools. States are responding to public demand for more information about school quality and student success that goes beyond a one-time test score. This development is made possible by years of state and federal investments in using longitudinal student data to draw richer insights about teaching and learning. This data will be published on school report cards, which means that families, communities, and policymakers will have more information about student progress than ever before. To make sense of what the data on school report cards tells us about student success and school quality, and use it to inform decisions that improve student outcomes, everyone--from parents to policymakers--needs to be able to understand what their state is measuring and what it means about students' academic progress. To offer some clarity, this brief provides an introduction to the different ways states will measure student growth in their ESSA accountability systems to evaluate school quality and support improvements.