The field of spoken language processing is concerned with creating computer programs that can understand human speech and produce human-like speech. Regarding the problem of understanding human speech, there is currently growing interest in moving beyond speech recognition (the task of transcribing the words in an audio stream) and towards "machine listening"--interpreting the full spectrum of information in an audio stream. One part of machine listening, the problem that this thesis focuses on, is the task of using information in the speech signal to infer a person's emotional or mental state. In this dissertation, our approach is to assess the utility of "prosody", or manner of speaking, in classifying speaker affect. Prosody refers to the acoustic features of natural speech: rhythm, stress, intonation, and energy. "Affect" refers to a person's emotions and attitudes such as happiness, frustration, or uncertainty. We focus on one specific dimension of affect: level of certainty. Our goal is to automatically infer whether a person is confident or uncertain based on the prosody of his or her speech. Potential applications include conversational dialogue systems (e.g., in educational technology) and voice search (e.g., smartphone personal assistants). There are three main contributions of this thesis. The first contribution is a method for eliciting uncertain speech that binds a speaker's uncertainty to a single phrase within the larger utterance, allowing us to compare the utility of contextually-based prosodic features. Second, we devise a technique for computing prosodic features from utterance segments that both improves uncertainty classification and can be used to determine which phrase a speaker is uncertain about. The level of certainty classifier achieves an accuracy of 75%. Third, we examine the differences between perceived, self-reported, and internal level of certainty, concluding that perceived certainty is aligned with internal certainty for some but not all speakers and that self-reports are a good proxy for internal certainty. [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.]