The VIDEO-LM project (Viewing, Investigating and Discussing Environments of Learning Mathematics) is aimed at enhancing mathematics teachers' reflection on their professional practice, through watching and discussing videotaped lessons of unknown teachers. The discussions are guided by the use of an analytic framework, comprised of six detailed "viewing lenses." In this article, we first provide a brief historical context of practicing teacher education, followed by the description of the framework we developed. We then present selected data from 17 implementation sites that illustrate the impact of VIDEO-LM courses on the professional growth of participants. Findings show that using the six-lens framework in video-based peer conversations supports the development of a reflective language, with which mathematics teachers engage in deep discussions about core issues of their profession.