Background: Migrant health is receiving increasing international attention, reflecting recognition of the health inequities experienced among many migrant populations and the need for health systems to adapt to diverse migrant populations. In the Greater Mekong Subregion (GMS) there is increasing migration associated with uneven economic integration and growth, socio-economic vulnerabilities, and disparities between countries. There has been limited progress, however, in improving migrant access to health services in the Subregion. This paper examines the health needs, access barriers, and policy responses to cross-border migrants in five GMS countries. Methods: A review of published literature and research was conducted on migrant health and health service access in Cambodia, Lao People's Democratic Republic, Myanmar, Thailand, and Viet Nam, as well as analysis of current migration trends and universal health coverage (UHC) indicators in the Subregion. The review included different migrant types: i.e. migrant workers, irregular migrants, victims of trafficking, refugees and asylum seekers, and casual cross-border migrants. Results: There is substantial diversity in the capacity of GMS health systems to address migrant populations. Thailand has sought to enhance migrant health coverage, including development of migrant health policies/programs, bilateral migrant worker agreements, and migrant health insurance schemes; Viet Nam provides health protection for emigrant workers. Overall, however, access to good quality health care remains weak for many citizens in GMS countries let alone migrants. Migrant workers - and irregular migrants in particular - face elevated health risks yet are not adequately covered and incur high out-of-pocket (OOP) payments for health services. Conclusions: UHC implies equity: UHC is only achieved when everyone has the opportunity to access and use good-quality health care. Efforts to achieve UHC in the GMS require deliberate policy decisions to include migrants. The emergence of the UHC agenda, and the focus on migrant health among policy makers and partners, present an opportunity to tackle barriers to health service access, extend coverage, and strengthen partnerships in order to improve migrant health. This is an opportune time for GMS countries to develop migrant-inclusive health systems.