Marketers have long recognized the power of word-of-mouth communication to influence consumer brand perceptions. Social media channels such as Facebook and Twitter make possible an efficient spread of communication to potentially large audiences with the added value of the credibility afforded to earned media. Consequently, marketers seek to encourage social media users to share brand-related messages. But how? To answer this question, we must first understand the decision to share or not to share in a social media context. This paper reports on an investigation as to the source and content of a brand’s tweets as antecedents of an individual’s decision to share that tweet among his/her followers. Our data show that both source and content interact to influence the share decision. Implications and future research are discussed.