A review of the literature shows that the wearing of a mask has been hypothesised to bring about four main psychological effects: disinhibition, transformation, facilitation of the expression of aspects of the wearer's Self, and various psycho-somatic changes. Several different explanations have been proposed as to why each of these effects come about. Using theoretical and empirical research, the thesis explores in detail the hypothesis that a mask can disinhibit its wearer, and that this disinhibition comes about because the mask-wearer feels less identifiable. The findings show that a mask can significantly reduces its wearer's feelings of identifiability, and that it can also significantly reduce its wearer's public selfawareness as a consequence of changes in attentional focus. However, the empirical evidence suggests that the mask's disinhibiting effect is limited to situations in which an individual wants to behave in a particular way, but inhibits that behaviour out of a concern with 'maskable' facets of their public self. Concomitantly, the findings suggest that, if an individual wants to behave in a way for which they require 'mask-able' facets of the public self, then the wearing of a mask may be experienced as inhibiting. This thesis also examines the hypothesis that a mask can transform its wearer, and that this occurs through the self-attribution process outlined by Kellerman and Laird (1982). The thesis provides strong empirical support for both these hypotheses, showing that the wearing of a mask can make individuals feel less like their usual self and more like the character represented in the mask. However, the empirical evidence suggests that this latter effect only occurs under conditions in which an individual is specifically focused on their masked appearance. A final chapter discusses the theoretical and applied implications of these findings, with specific reference to the use of masks in therapeutic practice.