This dissertation takes up the issue of transparency and opacity in vowel harmony--that is, when a segment is unable to undergo a harmony process, will it be skipped over by harmony (transparent) or will it prevent harmony from propagating further (opaque)? I argue that the choice between transparency and opacity is best understood as a competition between potential harmony triggers--segments are opaque when they themselves trigger spreading of the opposing feature value, and transparent when they do not. The analysis pursued in this dissertation is situated in the framework of Serial Harmonic Grammar, a variant of Optimality Theory which combines the step-wise evaluation of Harmonic Serialism with the weighted constraints of Harmonic Grammar. I argue that harmony is driven by a positively defined constraint, which assigns rewards rather than violations. Preferences for locality and for particular segmental triggers are exerted via scaling factors on the harmony constraint--rewards are diminished for non-local spreading, and increased for spreading from a preferred trigger. Evidence for this proposal comes from a diverse range of vowel harmony languages, in particular those with multiple non-participating segments which display asymmetries in their amenability to transparency. Segments more likely to be treated as opaque are also independently better triggers--they can be observed to be strong triggers in other contexts, and they are perceptually impoverished along the spreading feature dimension, which means they stand to benefit more from the perceptual advantages conferred by harmony. This proposal is also supported by experimental evidence. Results of a nonce-word discrimination task and a phoneme recall task both support the claim that harmony is perceptually advantageous; the latter suggests that this advantage obtains even among non-adjacent segments, and I argue that permitting explicitly non-local representations in harmony does not require abandoning phonetic grounding. Evidence for a trigger competition approach comes from a nonce-word study on Finnish disharmonic loanwords, which showed that vowels which are better triggers are more likely to induce transparent harmony, and less likely to be treated as transparent themselves. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]