This paper examines ways in which varieties of Spanish and Portuguese spoken in the Americas have diverged significantly from their peninsular sources, and from each other, in the half-millennium since colonization. Some of this divergence is a consequence of spontaneous innovations in the New World varieties (e.g., ‘zheismo’ and ‘sheismo’ in Platense Spanish; emergence of the new 1pl pronoun a gente in Brazilian Portuguese). Historically, a significant driver of change was language contact, with indigenous languages, and especially with African languages. A suite of linguistic characteristics shared by Caribbean Spanish and Brazilian Portuguese indicate that both varieties were affected by the irregular transmission of these languages to the African population transported to these locations in the time of slavery.