The Swedish teaching profession has undergone extensive reforms over the past four decades, leading to variations in teacher competence and specializations. More recently, the teaching profession has also faced challenges related to declining status, as well as difficulties in recruitment and retention. These disparities contribute to inequalities in teacher distribution and a widening achievement gap, yet there is limited understanding of how variations in teacher competence affects student achievement. This thesis investigates how teachers’ professional competence and working conditions relate to student achievement in Swedish middle schools (grades 4-6). It analyses data from TIMSS and PIRLS, combined with Swedish register data, to explore relationships between teacher competence and student outcomes in mathematics, reading, and overall performance in grade 6. The thesis consists of an integrative essay and three empirical studies, each focusing on different aspects of teachers’ professional competence and working conditions. The essay evaluates various models and frameworks of teacher competence while also summarizing research in this field. Study I investigates how formal education, subject-specific specialization, and teaching experience relate to grade 4 mathematics achievement, finding that a latent construct of formal teacher competence is strongly related to student performance. Study II examines teacher qualifications, reading specializations, reading comprehension activities, and cognitive activation strategies in grade 4, exploring their short- and long-term associations with student achievement. Study III examines how teachers’ working conditions and school climate relate to their job satisfaction and student achievement. Findings highlight the importance of formal education in improving students’ mathematics performance and subject-specific expertise in enhancing outcomes in both mathematics and Swedish. A curvilinear relationship emerges between reading comprehension activities in grade 4 and student achievement in PIRLS and Swedish by grade 6. Additionally, cognitive activation strategies appear to have a positive cross-subject relationship. The findings also underscore the importance of a positive school climate for both teacher well-being and student success.