This chapter illustrates how the Khao Hinsorn community has deployed Community Health Impact Assessment (CHIA) as a means to engage in – and challenge – an expert-led Environment Health Impact Assessment (EHIA) that backed the construction of a coal-fired power station. Through the CHIA, the community successfully revealed analytical shortcomings in the EHIA, and in the process broadened the definition of legitimate knowledge considered within formal state-led decision-making processes. The chapter argues that CHIA has emerged as an important and strategic collective action response in Thailand, which has contributed towards social learning and community empowerment, and thus enabled the contestation of unequal power relations within knowledge production, with implications for social justice outcomes. EHIA has adhered closer to the formula of existing EIA procedures, in terms of being expert-led, funded by project proponents, and "consulting" the public. Meanwhile, CHIA has emphasized community-led collective action research processes to co-produce knowledge with state and civil society experts.