The concept of Communities of Practice (CoPs) has tended to be employed by teachers who are comfortable working in a relatively informal arrangement for sharing professional knowledge within and among schools. There is evidence in several countries that the concept is undergoing change to become a serious strategy in the transformation of schools, defined as significant, systematic and sustained change that results in high levels of achievement for all students in all settings. Local, national and international networks are taking on the characteristics of powerful CoPs as teachers and their leaders share knowledge, solve problems and pool resources. Findings are drawn from 37 workshops in four countries over 18 months in 2005 and 2006. Critically important is alignment of four forms of capital to resource the effort. New forms of leadership are described.