​Internet advertising has come off age; yet little is known in research and practice about how digital channel advertising really works. The empirical research in this thesis intends to fill this gap and shed light on the effectiveness of online advertising. Two studies are conducted that focus on multichannel online advertising and search engine advertising, the single-most important online ad channel. In an interdisciplinary approach, both studies first develop comprehensive theoretical models based on existing work in related research fields—for example, marketing and information retrieval. This approach pays off and leads to new and insightful findings: - There are synergies in multichannel online advertising: purchase propensity increases when consumers receive advertising messages through multiple channels. - The channel order can influence the conversion probability. - Click-through rates in search engine advertising are influenced through various keyword criteria on semantic and syntactic level The results of this thesis constitute an important starting point for future research in online advertising. Furthermore, the results enable practitioners to improve the effectiveness of online advertising through a more differentiated campaign management approach. Based on its findings, the thesis outlines how a future integrated approach to online advertising could look like.