In this dissertation, I examine the nature and structure of consciousness. Conscious experience is often said to be phenomenally unified, and subjects of consciousness are often self-conscious. I ask whether these features necessarily accompany conscious experience. Is it necessarily the case, for instance, that all of a conscious subject's experiences at a time are phenomenally unified? And is it necessarily the case that subjects of consciousness are self-conscious whenever they are conscious? I argue that the answer to the former is affirmative and the latter negative. In the first chapter, I set the stage by distinguishing phenomenal unity from other species of conscious unity. A pair of conscious states is phenomenally unified if they are experienced together as part of a single experience that encompasses them both. In this and the next two chapters I defend the thesis that, necessarily, for any subject (of conscious mental states) at any time, all of that subject's conscious mental states (at that time) are part of a single, maximal state of consciousness. I call this thesis the "Unity Thesis." I proceed by considering some preliminary questions that might be raised about the Unity Thesis. For instance, the thesis presupposes that it is coherent to talk about parts of mental states. I consider objections by Tye and Searle and argue that the notion of an experiential part is unproblematic. In the remaining pages of the chapter, I present the source of the biggest challenge to the Unity Thesis: the data gathered from split-brain subjects. The Unity Thesis is formulated using the notion of a maximal state of consciousness. In the second chapter, I attempt to precisify this notion in a way that does not pre-emptively decide the debate over the Unity Thesis. In informal terms, a maximal state of consciousness is a sum of conscious states that are i) simultaneous, ii) have the same subject, and iii) all have a conjoint phenomenology. I call this the Consensus View. I then consider two unorthodox views that the Consensus View does not take off the table: the views that a "collective consciousness" and a "spread consciousness" are possible. A collective subject is one that can enjoy the experiences of an indeterminate number of "lesser" subjects of consciousness by sharing them together with those subjects. A spread subject is one that can enjoy the experiences of an indeterminate number of lesser subjects of consciousness, but it does so, not by sharing those experiences with the lesser subjects, but by absorbing the lesser subjects of experience into itself, thereby erasing the traditional boundaries between the entities we intuitively think of as subjects of experience. I argue that, although the Consensus View does not decide against them, these views stretch the bounds of coherence and should not, therefore, be accepted. Having presented an account of what maximal state of consciousness is, I define a stream of consciousness in terms of a maximal states of consciousness. In the rest of chapter two, I consider and argue against a number of different ways of interpreting the split-brain data that are either inconsistent with the Unity Thesis or attribute more than one subject of consciousness to split-brain subjects. Among the views I consider are Lockwood's partial-unity view and the views, by theorists such as Sperry, Koch, Puccetti, Marks, and Tye, that split-brain subjects have two non-overlapping streams of consciousness. In chapter three, I consider a recent attempt by Bayne to account for the split-brain data in a way that does not attribute two streams of consciousness to them. According to Bayne's Switch Model, the consciousness of split-brain subjects can be likened to that of a ball that is passed back and forth between the two hemispheres of the upper-brain. The hemispheres take turns supporting a single stream of consciousness. I consider the empirical data in some detail and argue that the data is not as compatible with the Switch Model as Bayne claims. I close the chapter by presenting the rough outline of an interpretation of the split-brain data that is consistent with both the Unity Thesis and the split-brain data. In chapter four, I turn from defending the Unity Thesis to examining an attempt to account for conscious unity. Rosenthal has offered a theory of conscious unity as an extension of his higher-order theory of consciousness. I consider his account of conscious unity in light of a well-known objection to his theory: the (Representational) Mismatch Objection. It can be asked what it is like for a subject of experience when a higher-order state misrepresents its target first-order state. If what it is like for the subject corresponds to the content of the higher-order state, then it appears as though higher-order representation is unnecessary for conscious experience, for it would appear as though it is possible for a state to be conscious without being represented by a higher-order state. If what it is like corresponds to the content of the lower-order state, then it would again seem as though representation at the higher-order level is unnecessary for conscious experience, for the higher-order state would not seem to be doing any work in generating the experience. I consider and argue against two recent defences of Rosenthal's higher-order theory from the Mismatch Objection. Then I turn to Rosenthal's account of conscious unity. Rosenthal's account posits two mental mechanisms. I refer to the ways of accounting for conscious unity via these two mechanisms as the "gathering strategy" and the "common-ascription strategy" respectively. Both of these strategies, I argue, appear to locate the basis for certain phenomenal facts in higher-order representational facts. This raises a prima facie question: does Rosenthal's account of conscious unity land him square within the sights of the Mismatch Objection? Although the gathering strategy may ultimately be understood in a way that does not make it subject to the Mismatch Objection, Rosenthal has certain commitments that bar this strategy from serving as a complete account of conscious unity. This is problematic for Rosenthal, I argue, because his common-ascription strategy faces some difficult questions. This strategy makes conscious unity due to an implicit expectation a subject of consciousness has that, whenever he or she engages in introspection, an explicit sense of conscious unity will be generated. I argue that it is very difficult to see how such an implicit sense could both avoid the Mismatch Objection and do the work it needs to do in order to account for conscious unity. In chapter five, the discussion turns from the unity of consciousness to self-consciousness. The question that is considered in this and the last chapter is the question whether conscious experience is necessarily accompanied by self-consciousness. The affirmative answer to this question I call the Ubiquity Thesis. I spend some time distinguishing robust conceptions of self-consciousness from minimal conceptions of self-consciousness. The notion of self-consciousness invoked by the Ubiquity Thesis is a minimal one. In spite of the fact that the Ubiquity Thesis invokes only a minimal or thin conception of self-consciousness, I believe the thesis to be false and argue against it. In this chapter I take up the views of Husserl. Husserl is often regarded as the progenitor of the phenomenological tradition, a tradition in which many philosophers affirm the Ubiquity Thesis. I examine and argue against an interpretation of Husserl's work, one defended by Zahavi, according to which Husserl could be seen to defend the Ubiquity Thesis. One claim that Husserl makes is that, in order for an object to become the intentional target of a conscious state, it must be given to consciousness beforehand. It is possible, during acts of deliberate introspection, for consciousness to take itself as its object. On Husserl's view, this requires consciousness to be given to itself beforehand. This self-givenness of consciousness, argues Zahavi, can be seen as a kind of minimal self-consciousness. Husserl has also offered an account of this self-givenness of consciousness and it appears in his discussion of inner time-consciousness. I attempt to argue, using some of Husserl's other views regarding psychological stances (or standpoints), that consciousness is not given to itself outside of the adoption of a certain psychological standpoint. I also offer an alternative way of accounting for inner time-consciousness, one that does not have, as a built-in feature, that consciousness always has itself as a secondary object. In the sixth and final chapter, I take up a contemporary defence of the Ubiquity Thesis. Kriegel, a higher-order theorist like Rosenthal, has argued that every conscious state is conscious in virtue of the fact that it represents itself. This self-representation is understood as a kind of self-consciousness and, thus, his theory can be seen as affirming the Ubiquity Thesis. In the first part of the chapter, I take issue with the way in which Kriegel lays out the conceptual terrain. In particular, Kriegel countenances a property he calls "intransitive state self-consciousness." I argue that this way of speaking is confused. I then turn to considering Kriegel's account. Kriegel identifies the species of self-consciousness that pervades all of conscious experience with a peripheral awareness of one's own mental states. I argue that such a peripheral inner awareness does not accompany all of our mental states and, thus, that Kriegel's views do not give us reason to accept the Ubiquity Thesis.