Many poor and oppressed people wish to leave their countries of origin in the third world to come to affluent Western societies. This essay argues that there is little justification for keeping them out. The essay draws on three contemporary approaches to political theory — the Rawlsian, the Nozickean, and the utilitarian — to construct arguments for open borders. The fact that all three theories converge upon the same results on this issue, despite their significant disagreements on others, strengthens the case for open borders and reveals its roots in our deep commitment to respect all human beings as free and equal moral persons. The final part of the essay considers communitarian objections to this conclusion, especially those of Michael Walzer.