In this study we consider the efficacy of a relatively new and widely accepted certification system for teachers established by the National Board for Professional Teaching Standards (NBPTS). To address the limitations in past research on the subject, we utilize a unique database covering the universe of teachers and students in Florida for a four-year span to determine the relationship between NBPTS certification and the impact of teachers on student test scores from both low-stakes and high-stakes exams. Contrary to some previous studies, we find evidence that NBPTS certification provides a positive signal of a teacher's contribution to student achievement only in a few isolated cases. Our results do reinforce evidence from previous research that the process of becoming NBPTS certified does not increase teacher productivity. While there is some evidence that NBPTS-certified teachers who are paid to act as mentors enhance the productivity of their colleagues, the effectiveness of non-NBPTS certified teachers does not increase with increases in the total number of NBPTS-certified teachers in the same school. (Contains 22 tables and 30 footnotes.)