This correlative study examined the role of teacher qualities and working conditions in 4th- and 8th-grade Turkish students' mathematics achievement in TIMSS 2019. Teacher qualifications were defined based on the teacher questionnaire used in TIMSS 2019 and were discussed in three categories: personal characteristics, teacher qualifications, and teacher practices. Data were analyzed using multilevel regression analysis. According to the results, working conditions explained most of the variance in the achievement scores (49% in the 4th-grade and 40% in the 8th-grade), while teachers' characteristics explained the least variance (19% in the 4th-grade and 11% in the 8th-grade). Teacher qualifications explained about one-third of the between-schools variance (35% in the 4th-grade and 26% in the 8th-grade). Teacher practices explained the one-fifth of the between-schools variance (23% in the 4th-grade and 27% in the 8th-grade). Some variables had a high correlation with TIMSS achievement in 4th and 8th-grade, such as teachers' age, experience, teaching limited by students not ready for instruction, and parental pressure on teachers. Other significant predictors were having a major in education and mathematics, bringing interesting materials to class, using long-term assessment projects, having too many administrative tasks, and the number of students in the class.