What is second language fluency? What is a foreign accent? Is it possible for an adult second language learner to speak fluently with a heavy accent or vice versa? What factors contribute to the perception of fluency and a foreign accent? What acoustic attributes correlate with the perception of fluency and a foreign accent? To answer these questions, speech samples were randomly selected from two spontaneous speech corpora produced by Mandarin native speakers and learners (both heritage and English learners of Chinese). Around 400 speech samples were assessed by 43 untrained Mandarin native speakers in Taiwan. Eight rating questions that reflected the oral proficiency of fluency, nativeness, accentedness, disfluencies, pronunciation, grammar, vocabulary, and comprehensibility were evaluated. An acoustic analysis followed that examined the acoustic attributes contributing to the perception of fluency and foreign accent. The findings of perceptual ratings showed that the disfluency rating highly correlated with the vocabulary score and vocabulary size in L2 production. This suggests that L2 fluency relates to the lack of vocabulary. The perception of accent correlated well with the pronunciation rating. Due to the strong correlations among rating variables, a principal component analysis (PCA) was submitted to reduce dimensionality and in order to reveal the internal structure of the data. The result revealed that all the rating variables contributed similar weights to the first principal component. The second principal component classified the rating variables into two categories. One group consisted of the knowledge factors of fluency, disfluency, grammar, vocabulary, and comprehensibility. The other group included the sound-related factors of nativeness, accentedness, and pronunciation. The acoustic measures loaded on the PCA rating platform demonstrated that the rate of speech and the second formant (F2) of vowels were the most powerful predictors of the perception of fluency and foreign accent, respectively. In the vowel studies, all phonetic vowels in Mandarin in the complete syllable inventory were investigated in terms of articulation and acoustics. The findings demonstrated that Mandarin [i, i-, [close back unrounded vowel]] pose different tongue positions, while the formant values of [i-, [close back unrounded vowel]] are not significantly different. In addition, the consonantal contexts influenced the articulation of vowels greatly. The comparison of vowel similarities between Mandarin and English indicated that the primary difference lies in F2. Vowels in Mandarin are further back (lower F2) than that in English. In the spontaneous data, the production of [u] by learners demonstrated the effect of L1 transfer in L2 speech. The [y] production is closer to [u], suggesting that the constraint of [+back] and [+rounded] is strong in English and difficult to disassociate. It is difficult to learn Mandarin [i-, [close back unrounded vowel]] because learners need to sustain their tongue position from the preceding consonants to vowels and must learn not to move their tongues during the articulation of vowels. Non-target-like production of Mandarin [a] and [alpha] resulted from the influences of coarticulation effects and transcription confusion. The findings suggested that segmental similarity is not the only factor in predicting L2 sound production. This thesis integrated studies combining perceptual ratings, acoustics and articulation to demonstrate a detailed mapping relationship between speech perception and speech production. The findings advance our understanding of second language fluency and foreign accent and have implications for both language teaching and language testing. [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.]