This dissertation investigated how four factors -- the degree of L2 experience, L1 sound structure, age difference in L1 language development and age of L2 exposure -- affect the perceptual cue weighting of duration and spectral differences for English tense-lax vowel contrasts. Four major hypotheses were tested: desensitization hypothesis (DH) (Bohn, 1995) and L1 transfer in L2 speech perception, the developmental weighting shift (DWS) (Nittrouer & Miller, 1997) in L1 language development, and the critical period hypothesis (CPH) (Lenneberg, 1967) in L2 language development. Participants identified synthesized English /i/ and /?/ vowels differing in six acoustically equal duration and spectral steps. Identification experiments designed to test the claims made by these above hypotheses were conducted with two groups of English language learners, Mandarin L2 learners and Arabic L2 learners. English native speakers were also included as control groups. Findings reveal that rich L2 input can make L2 learners (Mandarin and Arabic) become much more sensitive towards spectral differences, resulting in native-like speech perception, yet we did not find large duration-sensitivity shown by L2 learners as argued by DH. L1 sound structure has much more influence on L2 learners with limited English experience (Mandarin vs Arabic), but it has little effect with experienced L2 learners. As for L1 language development, no significant age-related differences were found between English native children and adults, contra expectations of the DWS. Lastly, L2 children (Mandarin and Arabic) with earlier L2 exposure but much less English input performed similarly with experienced L2 adults (Mandarin and Arabic), demonstrating that start age of learning an L2 is important and providing some indirect evidence for CPH. These studies together show that three of the four factors, the degree of L2 experience, L1 sound structure, and age of L2 exposure, interplay in the perceptual cue weighting patterns of English tense-lax vowel contrasts. Native-like performance can be achieved through rich L2 input or possibly learning an L2 at a younger age. Through examining English tense-lax vowels, this dissertation thus provides a better understanding of how L2 speech perception can be complex due to the interplay of multiple factors. [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.]