Historically, students took Algebra I in high school, but there has been a recent trend toward taking it in middle school (Domina, 2014). In the past two decades the proportion of middle school students taking Algebra I or more-advanced math courses has doubled (Domina, 2014). Success in Algebra I is important because it is a gateway course for advanced math (Star et al., 2015). Students who take and pass Algebra I are more likely to take and pass more-advanced math courses in high school (Snipes & Finkelstein, 2015). This trend has created a need for middle school teachers with more-advanced knowledge of math content, because prior research has suggested that teachers' knowledge of math content plays an important role in student achievement (National Mathematics Advisory Panel, 2008). The current study examines associations between the qualifications of middle school Algebra I teachers and their students' math achievement. Specifically, the study focuses on teacher certifications to teach math, education background, and performance on certification exams, particularly those associated with math content knowledge. Missouri Department of Elementary and Secondary Education staff might use the findings from this study when determining certification rules for middle school Algebra I teachers. The findings might also be useful to education leaders in other states as they consider which teacher qualifications are important for student achievement in Algebra I. Key findings include: (1) Teacher performance on math certification exams and years of experience teaching math were the qualifications most strongly associated with middle school students' Algebra I achievement; and (2) Teacher performance on math certification exams and years of experience teaching math were also strongly associated with Algebra I achievement for students in under-represented subgroups (Black students and Hispanic students) and disadvantaged subgroups (students receiving special education services and students eligible for the national school lunch program).