Focus structure has a profound effect on language production and processing. Yet, despite that focus has so much influence on an utterance, it still remains unclear what the meaning of focus is. Some theories consider focus to be entirely pragmatic, having no influence on the truth conditions or presuppositions of a focused sentence. Other theories consider focus to have a semantic contribution, contributing either to the truth conditions or the presuppositions of a focused sentence, but even within these, there is disagreement. For instance, von Stechow (1981, 1982) developed a theory in which the meaning of focus is to divide an utterance into an assertion and a background. On the other hand, Rooth (1985) developed a theory in which the meaning of focus is to introduce alternatives to the focused sentence into the derivation. Much of the debate between these theories of meaning has so far centered around how adequately the theory can compositionally account for the congruence between questions and focus in their answers, truth conditional focus association effects with operators like only, and, occasionally, word order. The difficulty for research into focus is that several very different types of theories have been demonstrated to be equivalently capable of accounting for these patterns. This dissertation, then, will attempt to address the question of what focus means from a slightly different angle. Its goal will be to narrow the realm of possible meanings for focus by experimentally testing for one possible property of focus: contrast. Specifically, this dissertation uses five experiments to test whether comprehenders need to compare a focused proposition to other propositions in order to interpret it. It investigates three things: 1) It investigates whether focus accesses a set of alternatives to an utterance. It will be looking for evidence that hearers are using alternatives to interpret a sentence with a focused word. 2) It investigates whether the members of this set are contrasted with the actual utterance. If hearers are using a set of alternatives to interpret focus, it could be the case that they access the set without forming any opinions about its members or the case that they consider all members except the actual proposition to be less ideal or even false. 3) It investigates whether this set of alternatives is accessed as part of the semantic derivation (ex: assertion or presupposition) or as a separate pragmatic process. Furthermore, it investigates whether the presence of a set of alternatives is a part of the truth conditions of a focused sentence (ex: an assertion of the sentence) or a part of the definedness conditions of a focused sentence (ex: a presupposition of the sentence). This dissertation finds that focus has two important properties: focus accesses a set of alternatives and at least this part of its meaning is semantically encoded as either a presupposition, an expressive meaning, or a context definedness condition. Additionally, members of the set of alternatives can be true and are semantic relations of the focused word. Even the English it-cleft is non-exhaustive. This recommends against purely pragmatic accounts of focus, and demonstrates that any theory of focus must include access to a set of alternatives. [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.]