Of the many school choice initiatives spreading throughout the country, charter schools are by far the most common. As of January 2000, over 1,700 charter schools serving more than 350,000 students have been created in nearly 40 states. That these schools are popular among parents can be seen by a simple fact: approximately 70 percent report having waiting lists. However, it's critical to remember that despite the rapid growth of charter schools, nearly 90 percent of American children continue to be enrolled in "traditional" public schools. Since this distribution of students across sectors is likely to change only modestly in the foreseeable future, we believe that one of the most important issues of the many raised by the charter school explosion is the extent to which charter schools affect the behavior of traditional public schools by competing with them for students.