Although significant federal funds are tied to student assessment, public education in this country is the responsibility of individual states. Subsequently, it seems as if there are as many approaches to measuring student ability and achievement as there are states. We now live in an increasingly mobile society in which today's families move from state to state--often for economic reasons. Therefore, when a student crosses state lines and enrolls at a new school, he or she will likely face a new measure of ability and achievement. In fact, a 1998 report from the National Center for Education Statistics, titled "Status of Education Reform in Public Elementary and Second Schools: Principals' Perspectives;' cited high student mobility as one of the top obstacles in applying high standards to all students. Other barriers included teaching students who are at different levels, and relying on assessments that do not measure what students can do. Educators can begin to address many of these issues with the development and adoption of unified measures of student ability. A consistent scale for measurement of student ability allows school administrators to build longitudinal growth profiles for individual students. Assessment of student ability and achievement is an important part of our education system. When done well, it provides a critical tool for monitoring and encouraging individual student progress. By developing and adopting common scales for measuring core student abilities such as reading and math, assessments can be used in a meaningful way that will help students of all abilities learn and achieve in schools throughout the country.