The term ‘environmental shielding’ has been used to refer to a class of processes in which the phonetic realisation of a nasal stop depends on its vocalic context. In Chiriguano, for example, nasal consonants are realised as such before nasal vowels (/mã/ → [mã]), but acquire 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 realised 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.