Despite the importance of speech naturalness to treatment outcomes, little research has been done on what constitutes speech naturalness and how to best maximize naturalness in relationship to other treatment goals like intelligibility. In addition, previous literature alludes to the relationship between prosodic aspects of speech and speech naturalness, but little is known about what specific aspects of prosody relate to naturalness. This study examines acoustic correlates of prosody in the speech of individuals with dysarthria and the relationship of these correlates with speech naturalness ratings. Four speakers with dysarthria due to Parkinson's disease (PD) participated in the first experiment in this study. Recordings of the speakers included vowel prolongations, speech alternating motion rates (AMRs), oral readings of sentences and short paragraphs, and spontaneous speech. Acoustic measurements of the data included the following: jitter, shimmer, harmonic-to-noise ratio (HNR), articulation rate, phonation time, average syllable duration, syllable duration, long-term average spectrum (LTAS), fundamental frequency, intensity, and spectral balance. Over 60 students in Communication Disorders participated in experiment two. Students were presented with 436 speech samples taken from experiment one and asked to provide speech naturalness ratings on a 1-9 Likert scale, based on Martin et al. (1984). After rating speech samples, students listed perceptual cues associated with samples rated most and least natural and weighted each cue on a visual analog scale. The results showed that subjects in this study shared many common prosodic characteristics with PD subjects in the literature, including difficulty timing speech gestures and voicing, exhibiting disordered voice quality, having a reduced frequency range in more disordered speech, and producing perceptually monotone speech. The data on naturalness ratings showed that spontaneous speech was rated the least natural on average, while sentences from the short story were rated slightly more natural and individually read sentences were rated the most natural of all of the utterance types. Thirteen themes emerged from the perceptual cues collected: articulation, content, dialect, fluency, gender qualities, intelligibility, loudness, nasal resonance, other cues, prosody, speech rate, syntactic boundaries, and voice quality. Comparison of the two experiments reveals a complex relationship between acoustic measures, severity, and perceived naturalness. [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.]