The Policy Research Working Paper 10767 presents FTT-FLEX, a tool for analyzing flexible technology diffusion in data-poor countries. The tool aims to provide a realistic description of several phenomena related to technology adoption, including technology diffusion, investment decisions, technology cost evolution, and technology lock-in. FTT-FLEX is suitable for single-country or macroeconomic model applications and can be used to achieve substantial emission reductions through widespread low-carbon technology adoption. The findings and conclusions expressed in this paper are entirely those of the authors and do not necessarily represent the views of the International Bank for Reconstruction and Development/World Bank or its affiliated organizations.