Internet advertising market has grown rapidly over the past decade. Its annual revenue has reached around 21.1 billion in 2007, and more than half of Internet advertising is related to Web advertising. Web advertising affects the majority if not all web viewers who would encounter some Web ads at one time or the other when they are using the Web. With such a magnitude on investment and in influence, however, our understanding of Web ads is limited and fragmented. This study aims to provide a comprehensive picture of the state of research on Web advertising, especially its interaction with individual viewers. In this meta-analysis of more than sixty empirical research articles published from 1996 to 2007, we develop a framework of Web advertising research that can be used to demonstrate the research foci so far. Both dependent and independent variables can be classified with the framework. Our findings indicate that (1) empirical studies on Web advertising have considered features of four components and their interactions: viewer, Web Ad, ad host, and product/service being advertised. The impacts of Web ads have been found to be on viewers and viewers' interactions with ads, hosts, and products/services. (2) The number of empirical research of Web advertising has increased gradually in different academic disciplines; however, the number is very insignificant compared to the growing Web advertising market and the broad influence of Web ads. (3) The study efforts are scattered and the findings are inconsistent and inconclusive. We conclude the existing studies and point out future research directions in this area.