Environmental education (EE) pedagogy is grounded in a view of teaching as a "creative and dynamic process in which pupils and teachers are engaged together in a search for solutions to environmental problems." However it is not only the students who need support engaging in this search but the teachers--in creating dynamic, rigorous exploration of environmental issues. Ian Robottom (1987b, 1987c, 1987d) promotes professional development to support teachers' pedagogical approaches to EE, which differ from traditional teaching approaches. These include interdisciplinary planning, active investigation of local issues, and robust participation--with students--in activities around environmental improvement. This research explores environmentally sustainable school practices through a case study of how one school design, Expeditionary Learning (EL), provides teacher professional development. This professional development promotes environmental-based education that aims to support teachers in learning and teaching about ecosystems and the natural world and has as its goal an impact on student experience and work. Embedded within this study, and the focus of this article, are two case studies of teachers in urban schools developing and implementing science expeditions, a primary pedagogical practice in the EL school design. The goal of both expeditions was to engage students in exploring ideas of sustainability within their communities, families, and the larger world. (Contains 1 figure.)