The purpose of this thesis is to examine different conceptions of decoherence and their significance within interpretations of quantum mechanics. I set out three different conceptions of decoherence found in the literature and examine the relations between them. I argue that only the weakest of these conceptions is empirically well supported, and that the other two rely on claims about the structure of the histories (robust patterns within the wavefunction) which we occupy which require justification. \n \nI also examine the ways in which conceptions of decoherence are used to solve aspects of the quantum measurement problem and support modern interpretations of quantum mechanics. I focus particularly on Wallace's Everettian interpretation of quantum mechanics. I argue that while decoherence is generally successful in supporting this interpretation in a variety of ways, the very strong conception of decoherence on which he relies is itself difficult to justify. \n \nI consider a variety of possible approaches to justifying the use of this strong conception of decoherence and argue that many of them are either unconvincing, or rely on controversial cosmological claims. Finally, I suggest that the best way to justify the use of this strong conception of decoherence is by appealing directly to its indispensability to an otherwise very attractive interpretation of quantum mechanics.