This chapter surveys the impact of language contact on phonological systems. The phonology of one language may influence that of another in several ways, including lexical borrowing, rule borrowing, Sprachbund features, and interlanguage effects. Illustrations of these phenomena are drawn from interactions between English and French, Hawaiian, and Japanese at different historical periods; from Quichean languages; from Slavic-influenced dialects of Albanian; from Dravidian influences on Sanskrit; and from South African English, among other examples. The evidence indicates that language contact may lead to various changes in phoneme inventory, phonotactics, and rule inventory, or to no change at all. Analyses of the data argue against the view that language contact invariably involves simplification but suggest that markedness is an important notion in accounting for certain features of interlanguages.