Environmental Impact Assessment (EIA) is an issue of concern to governments, organized civil society groups, as well as business actors in the Mekong region. EIA and related forms of environmental assessments are being carried out throughout the region with varying levels of quality, legal frameworks, monitoring and compliance. Through a political economy approach, we seek to understand the interests and incentives among key stakeholders in each of the five Mekong region countries and propose ways that EIA processes can potentially be improved, with reference to hydropower and other infrastructure and development projects. The analysis is based on a collaborative research process carried out under the auspices of the Mekong Partnership for the Environment, a USAID-funded program implemented by Pact that aims to advance regional cooperation on environmental governance. We find that at present, EIA implementation is limited by numerous political economy constraints, some general across the Mekong region, others specific to one or more country contexts. Certain of these constraints can be addressed through a regional cooperative approach, while others will require longer-term changes in social and political dynamics to encourage uptake and impact and avoid possible blockage from entrenched interest groups.