The "phase" alternation in Rotuman is remarkable (and has attracted a good deal of previous attention) for two reasons. First, the shape differences between phases are quite diverse, involving resyllabification, deletion, umlaut, and metathesis. Second, the phase alternation produces prosodic structures that are otherwise unattested in this language, replacing simple (C)V syllables with closed and diphthongal ones. In this article, I argue that Optimality Theory (Prince and Smolensky 1993) helps to make sense of both these observations. I also go on to use these results to support some claims about the nature of templates and prosodic circumscription in the theory of Prosodic Morphology (McCarthy and Prince 1986).