This paper explores the relationship between teacher testing and teacher effectiveness using a unique data set that links teachers to their individual students. The findings show a positive relationship between some teacher licensure tests and student achievement. They also suggest that states face significant tradeoffs when they require particular performance levels as a precondition to becoming a teacher. Some teachers whom we might wish were not in the teacher work force based on their contribution toward student achievement are eligible to teach based on their performance on the tests; other individuals who would be effective teachers are ineligible. For example, the results suggest that upping the elementary teacher licensure test standard from the one currently used in North Carolina to the higher standard used in Connecticut would lead to the exclusion of less than 0.5 percent of the teacher work force estimated to be very ineffective teachers, but would also result in the exclusion of 7 percent of the teacher work force estimated to be effective teachers. (Contains 27 footnotes, 4 tables, and 1 figure.) [This article was produced by the University of Wisconsin Press, Madison, WI.]