Hesitations in speech marked by pauses, fillers such as er, and prolongations of words are remarkably common in most spontaneous speech. Experimental evidence indicates that they affect both the processing of speech and the lasting representation of the spoken material. One theory as to the mechanisms that underlie these effects is that filled pauses heighten listeners' attention to upcoming speech. For example, in the utterance: (1) She hated the CD, but then she's never liked my taste in er music The hesitation marked by the filler er would heighten listeners' attention to the post-dis fluent material (music) which would then be processed and represented differently to an equivalent stimulus in a passage of fluent speech. The thesis examines this proposition in the context of an explicit de finition of attention. The first half of the work investigates whether hesitations heighten two different aspects of listeners' attention: these are the immediate engagement of attention to post-dis fluent stimuli at the point they are encountered, and the continued attention to the representation of stimuli after they are encountered. In experiment 1, a speech `oddball' paradigm is used to show that event-related potentials (ERPs) associated with attention (MMN and P3) are affected by a preceding hesitation, indicating an immediate effect of hesitations on listeners overt attention. Experiments 2 and 3 use behavioural responses and eye-movements measures during a change-detection paradigm. These experiments show that there is also an effect on the listeners' attention to the post-dis fluent material after the initial presentation of the utterance. The second half of the thesis concerns itself with the timecourse of the attentional effects. It addresses questions such as: how long-lived is the attentional heightening and what is the attentional heightening trigger? Experiments 4{7 explore the relationship between the filler er and periods of silent pause that surround it. Behavioural (exp. 4{6) and ERP (exp. 7) results show that while extending the period of silence after the filler er does not affect the immediate engagement of attention, it will affect subsequent attention to the post-disfluent material: constituents that are not immediately preceded by the filler er are not attended to in an enhanced way. Together, these experiments confirm the proposition that hesitations heighten listeners' attention to upcoming speech. The thesis outlines the ways in which the components of this attentional heightening are differentially affected by interaction between the content and timing of the hesitations encountered. Attention has an important role to play in the processing of any stimulus. Using disfluency as a test case, this thesis illuminates its importance in language comprehension.