Among policy makers, there is an ongoing discussion about the need to improve undergraduate education in science and engineering. With many undergraduate students being taught by graduate teaching assistants (GTAs), it is important to explore the development of STEM knowledge for teaching by GTAs. This study follows ten GTAs as they participated in a GTA teaching program that included attending a "Scientific Teaching" course. Data collection consisted of open-ended questionnaires and concept maps. Analysis revealed that a majority of the GTAs employed more didactic, teacher-centered practices while having varied levels of development in the areas of student understanding and instructional strategies. Those GTAs with regularly scheduled opportunities to work directly with students increased their knowledge for teaching. Additionally, GTAs with lower levels of prior teaching experience and an expressed desire to improve their teaching had the highest levels of knowledge development. From this study, we suggest that GTA programs support GTAs by initiating the preparation for teaching early in the TA experience. Finally, we recommend that science departments place more emphasis on teaching by providing GTAs with additional science education coursework as well as structured opportunities to work directly with students and to practice making use of reformed teaching strategies.