The "No Child Left Behind" Act requires that states align K-12 assessments with their academic standards for what students should know and be able to do. Alignment is at the core of standards-based education. The theory is that student achievement will improve if all parts of the system pull in the same direction. Under such a system, curriculum materials, teacher professional development, and classroom instruction all reflect a state's standards. Tests designed to measure students' performance of those standards clearly signal what is expected and probe whether students are learning the desired content and skills. Research points to a central conclusion: Alignment needs to be improved. In some extreme cases, studies have found that alignment between state standards and tests match the tests used in another state. Recognizing that no test can reflect fully a state's expectations for its students and that some standards are difficult to measure easily given the constraints of most paper-and-pencil exams, researchers do not suggest that states limit their standards only to what is measurable. However, the closer the alignment between state standards and tests, the more likely it is that teachers will focus on the desired content and students will have a fair chance to show what they know.