The purpose of this study is to investigate the use of an online discussion forum to support practicing elementary school teachers' professional development in mathematics. The forum is structured around teachers' sharing, discussing, and reflection upon the teaching of elementary mathematics. Third grade teachers (N=14) from three different school systems took part in the online discussion forum. This study provided insight into not only how these teachers make sense of teaching mathematics, but also how they support and share with each other as a learning community. (Contains 36 references.) (ASK)