This study sought to identify components of an asynchronous online teacher professional development program, "Prime Online," that potentially affected participants' mathematical knowledge for teaching (MKT). Twenty-three third- through fifth-grade general education and special education teachers completed a yearlong online teacher professional development program focused on improving MKT, instructional practices for all learners (particularly those with disabilities), and practitioner inquiry. Latent growth modeling and focus group data indicated growth in participants' content knowledge and initial growth in knowledge of students from pretest to midtest, with a decline at the end of the program. Module components are described to highlight the online teacher professional development program structure and specific activities that potentially supported participants' growth. Mathematical modeling, engaging with practitioner-focused journals and websites, developer-constructed materials, classroom implementation, and reflection and discussion provided participants with the opportunities for professional development resulting in increased MKT.