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].