The validity and accuracy of television programme ratings are critical to media planners and broadcasters alike. Media planners use ratings to select programmes that will satisfy certain reach and frequency objectives, which in turn contribute to an advertising campaign's overall goals. Broadcasters deliver audiences to advertisers, and base programme scheduling and pricing decisions on ratings data. At present, ratings are delivered within 24 hours of viewing and do not include adjustments for time shift viewing. Time shifting occurs when a television programme is videotaped and replayed at a later date. Given that VCR penetration in New Zealand has increased to over 75 percent of households, it is clear that programme ratings may be higher than the current overnight ratings suggest. This thesis explored the extent and measurement of time shifting in New Zealand. More specifically, it used AGB McNair's people meter data to examine: the scale of time shifting, the current methods of measuring time shifting, and future methods of estimating time shift viewing. The study aimed to identify whether patterns of time shifting behaviour exist, and whether these patterns could be used to model more inclusive overnight ratings. The findings suggest that, although the overall effect of time shifting on programme ratings is small, some programmes have very high levels of time shift viewing, prompting the need to include time shift viewing in the overnight ratings. The main constraint impeding the inclusion of VCR ratings in the overnight ratings is the difficulty in estimating time shift audiences overnight. This study proposed a number of methods of estimating VCR ratings overnight, including the recording level adjustment method, the same day playback adjustment method, and the genre/station correction method. While further research is required to compare the predictive ability of the methods, in the meantime implementing any of the methods is likely to provide more accurate overnight estimates of total audiences.