Research Findings: This is an evaluation study of the effectiveness of an online early mathematics professional development program for early childhood teachers. The eight online courses, along with the Math at Home: Early Math Matters website and resources, introduce early childhood teachers to mathematical concepts such as math literacy, number sense, patterns, geometry, measurement, data collection and math processes. Over 9,711 early childhood professionals participated in this professional development program, and a sample of 2,332 participants who gave consent to use their data reported on their course experiences, took quizzes, and provided qualitative feedback. In addition, a subsample of 95 participants took pre- and post-surveys on their attitudes, confidence, beliefs, and knowledge in teaching early math. The descriptive statistics of the participants’ self-reported course experiences, regression analysis of program effectiveness, and the pair wised t-test analysis of the survey data showed that program participants had positive experiences with this online professional development program, and that engagement in the program was effective in positively impacting their attitudes, confidence, beliefs, and knowledge in teaching early math. The participants’ positive experiences were associated with their demographic characteristics and their use and engagement with the course materials. Practice or Policy: Implications for future professional development efforts are identified, including scalability and the use of online professional development to impact early math learning and practice. (author abstract)