Abstract: Cross-media advertising, advertising in which more than one medium platform is used to communicate related brand content, has become widespread. Several reasons for cross-media strategies can be distinguished: target group extension, complementary effects, repetition, and synergy. Media synergy -the added value of a medium as a result of the presence of another medium- is considered a possible main advantage of cross-media advertising. Four processes which may underlie cross-media synergy effects are described in this chapter: encoding variability, multiple source credibility, forward encoding, and backward retrieval. This chapter also discusses factors that influence the effects of cross-media advertising, such as fit, sequence, and the multitasking consumer.There is little media planning data for cross-media campaigns, because audience readership research has traditionally focused on each medium separately; thus, information about the overlaps between people’s use of various media is lacking. The chapter discusses new developments in audience readership research which are important for cross-media campaigns, as well as designs for cross-media effect research.