This chapter focuses upon undocumented migrants: people who enter (or attempt to enter) a nation without legal authorization, that is, lacking the requisite paperwork. The methods used to estimate the size of undocumented populations in different nations are explained. The major types of undocumented migrants are described, including: people who overstay their visas, a group that comprises a very large percentage of the undocumented migrants in some nations; migrants who are smuggled, with the different types of smuggling arrangements also described; and people who are trafficked, including a discussion of the ways trafficked migrants are exploited.