Adult second language (L2) learners often experience difficulty producing and perceiving non-native phonological contrasts. Even highly proficient bilinguals, who have been exposed to an L2 for long periods of time, struggle with difficult contrasts, such as /r/-/l/ for Japanese learners of English. To account for the relative ease or difficulty with which L2 learners perceive and acquire non-native contrasts, theories of (L2) speech perception often appeal to notions of similarity. But how is similarity best determined? In this dissertation I explored the predictions of two theoretical approaches to similarity comparison in the second language, and asked: (1) How should L2 sound similarity be measured? (2) What is the nature of the representations that guide sound similarity? (3) To what extent can the influence of the native language be overcome? In Chapter 2, I tested a "legos" (featural) approach to sound similarity. Given a distinctive feature analysis of Spanish and English vowels, I investigated the hypothesis that feature availability in the L1 grammar constrains which target language segments will be accurately perceived and acquired by L2 learners (Brown [1998], Brown [2000]). Our results suggest that second language acquisition of phonology is not limited by the phonological features used by the native language grammar, nor is the presence/use of a particular phonological feature in the native language grammar sufficient to trigger redeployment. I take these findings to imply that feature availability is neither a necessary, nor a sufficient condition to predict learning outcomes. In Chapter 3, I extended a computational model proposed by Feldman et al. (2009) to nonnative speech perception, in order to investigate whether a sophisticated "rulers" (spatial) approach to sound similarity can better explain existing interlingual identification and discrimination data from Spanish monolinguals and advanced L1 Spanish late-learners of English, respectively. The model assumes that acoustic distributions of sounds control listeners' ability to discriminate a given contrast. I found that, while the model succeeded in emulating certain aspects of human behavior, the model at present is incomplete and would have to be extended in various ways to capture several aspects of nonnative and L2 speech perception. In Chapter 4 I explored whether the phonological relatedness among sounds in the listeners native language impacts the perceived similarity of those sounds in the target language. Listeners were expected to be more sensitive to the contrast between sound pairs which are allophones of different phonemes than to sound pairs which are allophones of the same phoneme in their native language. Moreover, I hypothesized that L2 learners would experience difficulty perceiving and acquiring target language contrasts between sound pairs which are allophones of the same phoneme in their native language. Our results suggest that phonological relatedness may influence perceived similarity on some tasks, but does not seem to cause long-lasting perceptual difficulty in advanced L2 learners. On the basis of those findings, I argue that existing models have not been adequately explicit about the nature of the representations and processes involved in similarity-based comparisons of L1 and L2 sounds. More generally, I describe what I see as a desirable target for an explanatorily adequate theory of cross-language influence in L2 phonology. [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.]