Contextual and strategic factors that influence the use of endorsements in political advertisements represent an underexplored area of the scholarly literature, resulting in a gap between the art and science of campaigning and the advertising-effects literature. This article uses data from the Wisconsin Ad Project to study endorsements in ads aired during the 2008 election cycle. We show that endorsements are used strategically by campaigns and that four factors—candidate characteristics, campaign characteristics, ad format, and timing—affect whether an ad containing an endorsement is aired. Specifically, endorsements are more likely to appear in positive ads, are primarily used by incumbents and female candidates, are less likely to appear in competitive races, and tend to air less frequently as a campaign progresses.