Teachers are participating in online communities in ever increasing numbers to find and share knowledge with educators around the world. However, the majority of the studies on online communities of practice often fail to examine the process of knowledge sharing as a complex, dynamically evolving practice that is shaped by local classroom and school contexts as well as other sociocultural factors.

This study was designed to address the need for a more comprehensive and multifaceted exploration of teacher learning in an online community of practice. A Cultural Historical Activity Theory (Leontiev, 1978; Engestrom, 1987; Cole and Engestrom, 1993; Vygotsky, 1978) framework was used as a guide for examining how teachers acquired and made use of the shared knowledge from the Edmodo Math Subject Community, an online community of practice with more than 250,000 members. Multiple data types were collected in order to examine the teachers' object-oriented actions in two overlapping activity systems.

Data analysis revealed that the participants were driven by the contradictions and limitations of their local school activity systems to take control of their learning and find knowledge in the Math Subject Community. The participants' ability to find knowledge was defined by the roles they performed in the community, the tools they used, the collective knowledge of the community, and the implicit community rules of reciprocity and professionalism. The participants' ability to select and implement the knowledge they found in the Math Subject Community in their classrooms was shaped by their local school activity system tools, the support of their colleagues and administration, their students' range in math abilities, and the community rules.

A Model of Teacher Knowledge Acquisition was developed to display the participants' fluid, ongoing process of navigating between two overlapping activity systems to find new knowledge. The Model of Teacher Knowledge Acquisition presents teacher learning as a complex, socially constructed process that is influenced by multiple activity systems that interact with and shape one another.

Overall, the participants felt that engaging in this process of acquiring knowledge allowed them to make changes, both small and large, in their teaching and learning strategies.