The advertising industry has been characterised as a mechanism for controlling individuals and populations in line with the demands of corporate capitalism. This thesis interrogates the validity of those characterisations by examining the influence of new technologies on the production and dissemination of advertising. If advertising is a rational, rationalising practice, then decades of technological improvements suggest that contemporary advertising should be more predictable, calculable, and efficient than in previous generations. Using data drawn from a year-long ethnography complemented by a series of 32 semistructured interviews with industry practitioners, this thesis finds the industry undergoing a fundamental shift in response to the rise of digital media. The new media give advertisers access to unprecedented amounts of data, but the expertise required to make effective use thereof is scarce; the underlying algorithms are imperfect; and the measures used to evaluate efficiency show little correlation to actual consumer behaviour. Digital media have also contributed to a fragmentation of audiences, a problem for brand advertising, while offering consumers access to competing information about goods and services with which to publicly challenge the claims of advertisers. The data suggest that contemporary brand advertising - of the sort most commonly associated with the industry - is essentially non-rational. Practitioners are no more able to predict performance than at any other time in their history, and ads are produced according to artistic rather than scientific principles. Sales-based, programmatic advertising, however, is increasingly rational. This emerging tension suggests a functional split in the professional industry, with practitioners preferring to emphasise the (uncertain) value of creative brand advertising even as clients are increasingly diverting their budgets into more rational exercises in advertising. These findings suggest that the first casualty of a truly rational advertising system may well be the advertising industry itself.