Ensuring access to essential medicines is a key objective of all health systems, and is an integral component of the progress towards universal health coverage (UHC). Despite global and national efforts to improve access and affordability of medicines, millions of people - particularly in low- and middle-income countries - still remain without access to quality-assured and affordable medicines. This study aims to contribute to existing knowledge on regulatory systems and harmonization efforts in Southeast Asia. Focusing on five member states of the Association of Southeast Asian Nations (ASEAN) - Indonesia, Malaysia, the Philippines, Thailand, an Vietnam - this study gives an overview of pharmaceutical markets and key pharmaceutical policies in the region, provides a cross-country comparison of medicines regulatory systems, and details harmonization efforts, opportunities, and challenges.