Adult second language (L2) learners often experience difficulty producing and perceiving nonnative phonological contrasts. Even relatively advanced learners, who have been exposed to an L2 for long periods of time, struggle with difficult contrasts, such as /?/-/l/ for Japanese learners of English. To account for the relative ease or difficulty with which L2 learners perceive and acquire nonnative contrasts, theories of L2 speech perception and phonology often appeal to notions of "similarity", but how is "similarity" best captured? In this article, we review two prominent approaches to similarity in L2 speech perception and phonology and present the findings from two experiments that investigated the role of phonological features in the perception and lexical representation of two vowel contrasts that exist in English, but not in Spanish. In particular, we explored whether L1 phonological features can be reused to represent nonnative contrasts in the second language (Brown, 1998, 2000), as well as to what extent new phonological structure might be acquired by advanced late-learners. We show that second language acquisition of phonology is not constrained by the phonological features made available by the learner's native language grammar, nor is the use of particular phonological features in the native language grammar sufficient to trigger redeployment. These findings suggest that feature availability is neither a necessary, nor a sufficient, condition to predict the observed learning outcomes. These results are discussed in the context of current theories of nonnative and L2 speech perception and phonological development.