This study describes the development and validation of an instrument to measure graduate teaching assistants' (GTAs) learning about teaching during professional development. In the pilot study, exploratory factor analysis of data from 239 graduate students indicates a single factor structure. The second study, involving 177 science, technology, engineering, and mathematics (STEM) GTAs, confirms the single factor structure of the instrument. The instrument is highly reliable with both populations. The instrument is correlated to the hours STEM GTAs spend in professional development and their self-efficacy in teaching. It is sensitive to departmental differences between GTAs perceptions of their professional development. This instrument has multiple possible users including university faculty involved in GTA professional development as well as educational researchers. University faculty can use it for needs assessment during GTA program development, comparisons among departmental programs, and in improving current GTA programs. It also provides a sensitive measurement of the quality of GTA professional development in multiple program research studies.