Mandarin is a tone language that is spoken by billions of people in the world. Despite huge demand, there is a lack of assessment and treatment tools targeting Mandarin speakers with communication disorders. To better understand the speech profile of Mandarin speakers with communication disorders, this dissertation first examined contrastive stress, an aspect of prosody, in persons with Parkinson's disease (PD). It then explored a Mandarin tone training method for English-speaking learners of Mandarin, with the goal of taking a preliminary step toward a potential clinical application for Mandarin speakers. The dissertation consists of three studies. Study 1 investigated production and perception of contrastive stress in persons with PD who speak Mandarin. The results showed that participants with PD had poorer performance in producing contrastive stress compared to control speakers based on healthy listeners' rating. However, the PD group performed as well as the controls in perceiving contrastive stress. A differential task effect was also found, with the PD group exhibiting better production in a repetition task than in elicited speech. Study 2 analyzed acoustic cues of contrastive stress in persons with PD and examined the influence of speech task on these patterns. The results suggested that while the use of [fundamental frequency] was relatively compromised in the PD group, their use of duration cue was relatively preserved. In addition, the PD participants were less capable of using [fundamental frequency] and durational cues to signal contrastive stress in elicited speech, but they demonstrated similar acoustic patterns to controls in the repetition task. Study 3 measured the efficacy of real-time biofeedback in L2 tone production and examined how learners' baseline aptitude influenced learning outcomes. The results did not show a significant difference in production gains between training conditions, but learners' baseline aptitude was a strong predictor of post-training production accuracy for all tones. For Tone 4 only, an interaction between aptitude and training condition suggested that the biofeedback condition was more beneficial to learners with high aptitude. The findings of these studies will contribute to the evidence base for the development of assessment and intervention tools targeting communication disorders in tone language speakers. [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.]