Consumers today are in the focus of a wide range of companies in a more and more global world, all of which implies an increase of marketing activities and hence an increase of advertising effort. Hundreds or even thousands of advertising messages are put in front of consumers every day through various media channels. However, every targeted customer has a very limited ability of perceiving advertising messages and even a smaller percentage of what has been recognized is considered as being relevant. Therefore consumers quite often feel harassed and overloaded by the mass of information, the quality of advertising and the context in which it is shown, and react protective towards advertising, especially when they do not belong to the target group of the advertised product or service. This implies a great amount of budget wastage and a potential damage to brand images, since too many consumers are targeted with the wrong advertising message.Furthermore media consumption has changed over the years, while the consumption of TV for instance remains stable, the internet consumption increases dramatically, in the younger target groups it has already outgrown the TV consumption, and therefore an attenuation of this trend can be expected. This means more and more prospective consumers will spend more and more time online and are therefore important potential targets for advertising campaigns. Keeping in mind the negative consequences of the irrelevant and too intense advertising the question arises if there is a way to avoid the drawbacks of offline media and use the opportunities offered by online media. The necessity of defining the right target group for an advertising campaign is as undoubted as the difficulties implied in that process, the end product being able to target the right persons online. Behavioural targeting might offer a way to analyse online media and therefore to contribute to the requirements for optimal media planning in order to achieve a high degree of efficiency. Against the background of the increasing interest in behavioural targeting an examination of its potential importance for marketing will be conducted in this study in order to asses if behavioural targeting can increase the efficiency of media planning in online media.The foundation is provided by the description of media planning in general, its objectives including the target group segmentation, media strategy and selection programmes and concluding by summarizing the requirements for optimal media planning.In order to systematize the subject area of behavioural targeting an insight into online advertising is provided. This study aims to preserve an international scope, however due to the fact that the actual maturity of the online media industry is at very diverse stages worldwide, focus will be on the U.S. and selected countries in Europe such as Germany, France, the UK and Scandinavia.