Migration is a natural state of humankind and has been documented throughout history. Some people may flee violence and persecution, while others simply seek a better life. Although migration is often classified into these two basic categories, the reality is more complex and nuanced: people migrate for a myriad of interconnected cultural, economic, religious, ethnic, and political reasons. Depending upon the epoch, migration has been seen in a positive or a negative light. Currently, the terms migrant and refugee have become politically charged and are widely misused for political and populist purposes. However, no matter how migration is portrayed at a specific point in time, it will inexorably continue. Thus, the need to ensure the protection, health, and welfare of people on the move is imperative and provides the rationale for the accompanying PLOS Medicine Special Issue on Refugee and Migrant Health [1]. This imperative is not only a matter of humanity and equity but is also necessary for the global economy, as migration is inherently linked to economic growth [2]. The governance needed to provide health services to this diverse and widespread group of people-from low-waged migrant workers and undocumented migrants to refugees-is unclear. How can we attain universal health coverage in this complex and uncertain environment? At the 72nd World Health Assembly in May 2019, a global action plan was agreed upon that seeks to establish a "framework of priorities and guiding principles.. .to promote the health of refugees and migrants." The Global Compact on Migration, developed through intergovernmental negotiations and adopted in December 2018, enshrined health as a cross-cutting priority for migration governance. It is, however, unlikely that governments will apply such frameworks, unfortunately. Despite widespread recognition of the numerous migrationrelated health risks, mobile populations are often met with punitive border policies, arbitrary detention, abuse, and extortion and are denied access to healthcare. All too often, government policies prioritize the politics of xenophobia over their responsibilities to act forcefully to counter them. As human beings, migrants are entitled to universal human rights without discrimination, and to the "highest attainable standard of health" according to international law. Migration health remains at the margins of policy prioritization for most governments, and thus universal health coverage remains elusive for the vast majority of migrants and refugees [3]. Seeking to raise awareness of the health inequities and different contexts faced by migrants and forcibly displaced persons, as well as to promote research, service, and policy innovation in this area, this Special Issue is devoted to migrant and refugee health in the broadest sense.