In the Least Economically Developed Countries (LEDCs), School Based Teacher Development (SBTD) is sometimes advocated as a potential mechanism for improving the classroom practices experienced by millions of children in a complete school system, as quickly as possible. Robust evidence is required for approaches to be implemented with some confidence by Government development agencies, such as the UK Department for International Development (DFID). SBTD has a long history stemming from ideas of school-based curriculum development, which underlay the ideas on teacher-as-researcher, and is typically advocated in the developed world based on a view of the teacher as a professional. How might such notions play out, and to what effect, in LEDC contexts? This article examines the issues at stake in introducing SBTD in LEDCs, by examining: the nature of the evidence for various forms of teacher development, the nature of SBTD in particular and the evidence for its effectiveness. The latter issues will be illustrated through examination of English in Action (EIA, www.eiabd.com), a large-scale SBTD programme for primary and secondary English language (EL) teachers serving government schools across Bangladesh. (Contains 8 figures and 6 tables.)