This study concentrated on the theory-practice problem in mathematics teacher education. We examined 13 student teachers' use of theory when they reflected on teaching practice in a class specifically designed to optimize the chance for theory use. We developed a Reflection Analysis Instrument with which the student teachers' use of theory could be identified univocally and described in a systematic way. We describe a case study that illustrates the nature and level of one student teacher's use of theory and discuss variations in all of the student teachers' reasoning and differences in their depth of theory use.