With the increasing popularity and accessibility of the Internet and Internet-based technologies, along with the need for a diverse group of students to have alternative means to complete their education, there is a major push for K-12 schools to offer online courses, resulting in a growing number of online teachers. Using the Tailored Design survey methodology (Dillman, 2007), this study examines a national sample of 596 K-12 online teachers and measures their knowledge with respect to three key domains as described by the TPACK framework: technology, pedagogy, content, and the combination of each of these areas. Findings indicate that knowledge ratings are highest among the domains of pedagogy, content, and pedagogical content, indicating that responding online teachers felt very good about their knowledge related to these domains and were less confident when it comes to technology. Correlations among each of the domains within the TPACK framework revealed a small relationship between the domains of technology and pedagogy, as well as technology and content (0.289 and 0.323, respectively). However, there was a large correlation between pedagogy and content (0.690), calling into question the distinctiveness of these domains. This study presents a beginning approach to measuring and defining TPACK among an ever-increasing number of K-12 online teachers. (Contains 3 tables.)