Internet advertising (also known as online advertising) is the delivery of advertising messages and marketing communications through the Web. It has presented sustainable revenue growth since its inception in the mid-1990s. Internet advertising takes a multitude of forms, ranging from the “traditional” banners to today's rich media ads. Due to its wide success, internet advertising currently constitutes a significant marketing channel for a large number of firms, ranging from news agencies to car manufacturers and retailers, and it is thus gradually gaining attention in the context of marketing communications strategy planning and implementation. However, there has been relatively little empirical research and actionable knowledge on the factors that affect the success of online advertising. Consequently, when relying on insufficient knowledge to make decisions about their internet marketing strategy, companies run the risk of missing marketing opportunities or damaging their image. To this end, the work presented in the context of this thesis attempts to make a number of contributions to the body of actionable knowledge pertaining to the planning of internet advertising campaigns for a company. More specifically, this report aims at analyzing the impact of the type of online user activity as well as of the user’s online social context on the effectiveness of internet advertising. The objectives of the thesis are pursued through the design and implementation of an online experiment that simulates four types of online activities that are popular among today’s plethora of Web 2.0 applications. Such activities range from “traditional” ones, for example online article reading, to recently adopted, for instance photo tagging, social bookmarking and messaging. In addition to the role of online activity type, the experiment plans to investigate the impact of a user’s social context, that is the role of a user’s online social network on his/her receptiveness to internet advertising. Starting from the study of the most influential research works in the area of internet advertising effectiveness and following the principles of their experimental methodology, an online experiment was designed and implemented that collected input from a set of 87 users. The analysis of the obtained input reveals significant correlations between the type of online activity of users and the effectiveness of internet advertising. Furthermore, there is evidence that the content of a webpage and the degree of its congruency to the advertising content play a significant role on the impact of online advertising.