This study investigates the effect of a professional development program designed to develop prospective mathematics teachers' ability to notice students' proportional reasoning. It examines how those teachers attended to student thinking, how they interpreted it, and how they decided to respond. Sixteen prospective teachers (PSTs) at a state university participated in a seven-week online professional development (PD) program. We provided participants with a research-based framework for proportional reasoning and engaged them in various ways via interviews with middle school students. They also wrote a reflection paper and received feedback. Combining multiple elements with interviewing was a unique feature of this program. The improvement in participants' noticing skills was statistically significant. Specifically, in the post-assessment, the participants paid attention to most of the mathematical details of the students' strategies, provided interpretations supported by conceptual underpinnings and evidence, and made research-based student-oriented decisions that focused on key issues. Implications for future teacher education practices and suggestions for further research are provided.