The use of marketing communications by consumers as the basis of some form of social interaction is potentially exciting in that it suggests high levels of engagement with brands and brand messages. This chapter defines the concept of communication in a way that encompasses both process and outcome and considers its role within the context of marketing. Key communication models are identified, and their relevance to modern-day products and markets is discussed. The chapter examines the factors that either aid or inhibit the effectiveness of a piece of marketing communication and explores the reasons why this might be the case. A communication that is undifferentiated in both its message and medium of delivery is where each target consumer is subject to the same message and media combination; no modifications are made to take into account differences between target segments or, indeed, differences inside individual target groups.