The term "environmental shielding" refers to a class of processes in which the phonetic realization of a nasal stop depends on its vocalic context. In Chiriguano (Tupí; Dietrich 1986), for example, nasal consonants are realized as such before nasal vowels (/mã/ → [mã]), but acquire an an oral release before oral vowels (/ma/ → [mba]). Herbert (1986) claims that shielding protects a contrast between oral and nasal vowels: if Chiriguano /ma/ were realized as [ma], [a] would likely carry some degree of nasal coarticulation, and be less distinct from nasal /ã/. This article provides new arguments for Herbert’s position, drawn from a large typological study of South American languages. I argue that environmental shielding is contrast preservation, and that any successful analysis of shielding must make explicit reference to contrast. These results contribute to a growing body of evidence that constraints on contrast are an essential component of phonological theory.