Reliability is a core measurement issue that has taken on new meaning in this era of educational accountability and No Child Left Behind (NCLB). In this chapter, reliability is discussed in the context of NCLB requirements; in the context of measurement error, sampling error, and score volatility; and in the context of accountability systems and the probability of misclassifying schools. Following these discussions, several recent developments in evaluating the reliability of accountability systems, alternative approaches for better evaluating school effectiveness, and new models for gauging the success of all students are described.