This study examined the influence of professional development on elementary school teachers' perceptions of and use of an internet-based formative assessment tool focused on students' number sense skills. Data sources include teacher-participants' pre and post survey, open ended response on post survey, use of the assessment tool and their written responses completed during online professional development modules. Through an inductive analysis of teachers' written reflections and open ended survey response, participants reported that the use of the formative assessment tool supported their instruction, provided opportunities for teacher collaboration, and served as a vehicle for teachers' own professional learning. A quantitative analysis of the pre and post survey indicated a statistically significant increase in teacher practices to be more student centered. Implications for future research related to professional development focused on supporting teachers' internet-based tools are also shared.