This paper investigates the development of syllable structure in creoles. Most importantly, an analysis of vowel epenthesis in Sranan, both synchronic and diachronic, will be provided, and it will be shown that the present vowel harmony system evolved gradually from a default vowel system. The results are formalized in an optimality-theoretic analysis, and we argue that the epenthesis system is largely substrate-motivated, by comparing constraint rankings. Syllable structure constraints in other creole languages will also be discussed, and it will be shown that less radical creoles have syllable structure systems of intermediate complexity, similar to interlanguage systems. An optimality-theoretic approach to language acquisition as constraint reranking is employed to explain how such intermediate systems develop.