This study was conducted among 269 student teachers at 11 primary teacher training colleges in the Netherlands. To investigate their competence in integrating theory and practice in their reflections on mathematics teaching, a learning environment was designed to evoke theory use in reflections on practice. To be able to systematically describe the use of theory, we distinguished two dimensions, which we called the "nature" and "level" of theory use. A Reflection Analysis Instrument was used to univocally code the nature and level of the student teachers' theory use in the reflective notes of their final assessments into 1740 meaningful units. We found that nearly all student teachers used theory. However, they differed markedly in the way they linked theory and practice and with which depth they used theoretical concepts in their reflections. A remarkable finding of the study was the important influence of prior mathematics education on the nature and level of theory use, especially the low results of the third-year student teachers in their level of theory use. The outcome may have consequences for the design of the teacher education curricula and for the intake of first-year student teachers.