This article describes the work Colorado has done to meet the "highly qualified teaching" requirements of the federal No Child Left Behind Act. Colorado's standard is devoid of the loopholes that have drawn criticism to other states' policies, many of which, observers have suggested, fall short of both the letter and the spirit of the law's provision on "highly qualified" teachers. Colorado, in contrast, got the only "A" for its plan among 39 graded by a national group concerned with teacher quality. According to Dorothy Gotlieb, the deputy education commissioner who oversees teacher-qualification rules, the state's standard--which calls for experienced teachers to meet almost the same requirements as new teachers--is also a practical success. To be deemed "highly qualified," a Colorado classroom veteran has one option more than those available to a new teacher, who must either complete a college major or pass a test in the subject he or she teaches. The veteran can have the major, take the test, or accumulate 24 college credits or the equivalent in professional development in the relevant subject. That 24-credit rule falls six credits short of the state's definition of a major. Still, it equals what the licensing system allows in the case of a teacher receiving a license "endorsement" to teach a second subject. In effect, then, the state offers no alternative route for experienced teachers, the option known as a HOUSSE, for "high, objective, uniform state standard of evaluation." Federal law does not require one, and in fact, some advocates of improved teacher quality fought its inclusion. Colorado's rules do permit teachers to meet the "highly qualified" requirement via the HOUSSE route if a teacher can demonstrate subject-matter mastery with three years of student-achievement growth as shown by standardized-test scores. But in the two years that the state has been working on its plan, no formula has yet been devised for doing that. Many of Colorado's education leaders believe that rural areas will have the most trouble meeting the teaching requirements, and the most to lose as the federal deadline approaches.