Many languages have processes which lengthen or shorten a vowel or consonant. In this dissertation, I concentrate on Open Syllable Lengthening, Closed Syllable Shortening, Monosyllabic Lengthening and Trochaic Lengthening, and present a formal model which captures these lengthenings and shortening as the insertion, deletion or reassignment of a mora, and also investigates their prosodic motivations. Three case studies are presented which illustrate these lengthenings and shortenings and their interaction with syllable structure and other processes in two German dialects and one Germanic language. My analysis supports the view that geminates have to be underlyingly moraic and that final singletons have to be treated as extrametrical. Chapter 2: introduces the theoretical background for the dissertation and presents the moraic faithfulness constraints, as well as a basic set of prosodic markedness constraints necessary for a formal analysis. Chapter 3: focusses on Imst German, a dialect spoken in and around Imst, Austria. My analysis shows that Open Syllable Lengthening, Monosyllabic Lengthening and Closed Syllable Shortening neutralize contrastive vowel length in most contexts. It is preserved only before medial geminates which I analyze as faithfulness to underlying morae. Chapter 4: investigates Bernese German, spoken in an around Bern, Switzerland. Unlike Imst German, this dialect shortens vowels in open stressed syllables. I argue here that this is an instance of Trochaic Shortening, requiring an even trochee. My analysis shows that all other lengthenings and shortenings are motivated by the same requirement as well. Contrastive vowel length is preserved only before medial geminates to minimize moraic faithfulness violations when an even trochee is not possible. Chapter 5: consists of an analysis of Icelandic, in which vowel length is entirely predictable but geminates are phonemic. I show here that Open Syllable Lengthening, Closed Syllable Shortening and Monosyllabic Lengthening interact with the syllabification of medial consonant clusters and preaspiration based on sonority distance in syllable contact and the avoidance of moraic aspirated stops. Chapter 6: summarizes the dissertation and provides directions for future research. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]