In this first book-length study of synchronic umlaut, a comprehensive comparative analysis of the phonology and morphology of the umlaut alternation in present-day German and the Austronesian language Chamorro is presented in the framework of Optimality Theory. A fresh perspective of the phonology-morphology interface and the interaction between segmental and metrical structure with wider cross-linguistic implications is developed, including a new conception of morphological conditioning based on morphological faithfulness and Representation as Pure Markedness. The Chamorro data collected for this study contribute significantly to the documentation of this endangered language.