While the United States historically has been a polyglot nation characterized by great linguistic diversity, it has also been a zone of language extinction in which immigrant tongues fade and are replaced by monolingual English within a few generations. In 1910, 10 million people reported a mother tongue other than English, notably German, Italian, Yiddish, and Polish. The subsequent end of mass immigration from Europe led to a waning of language diversity and the most linguistically homogenous era in American history. But the revival of immigration after 1970 propelled the United States back toward its historical norm. By 2010, 60 million people (a fifth of the population) spoke a non-English language, especially Spanish. In this essay, we assess the effect of new waves of immigration on language diversity in the United States, map its evolution demographically and geographically, and consider what linguistic patterns are likely to persist and prevail in the twenty-first century.