Fewer than 20 states require middle grades certification; in most states, credentials overlap so that both elementary and secondary certified teachers may teach in the middle grades. Moreover, in many urban districts, getting teachers for the middle grades is a challenge. Despite this crazy-quilt of certifications and a growing body of work on teacher qualifications, there have been few studies that examine the teachers' impact on learning in the middle grades. This paper uses a data set from an urban district to estimate the impact of different certifications (and lack of certification) on middle-grades students' learning gains in mathematics and science. In mathematics, we find that students with elementary- and secondary-certified teachers outscore those with uncertified teachers and those who are certified in special education. Especially strong effects are seen in science, where students with secondary science-certified teachers substantially outscore those with any other kind of teacher. (Contains 3 tables and 3 notes.)