Social television combines traditional television viewing and interactions with social media to create a phenomenon that connects otherwise autonomous viewers through a shared viewing experience. This dissertation explores one type of social television: on-screen user-generated comments. Although the practice spans multiple television genres, little is known about its effect on viewers’ cognitive processing of the media, perceptions of the social presence of other viewers, or the viewers’ experience of the media. Two experimental studies explored the effects of on-screen user-generated comments on cognitive processing of the media message, the effect of manipulating the content of on-screen user-generated comments and individual differences on perceptions of social presence, and how the perception of social presence affected both the viewing experience (i.e., enjoyment of and engagement in the television show) and cognitive processing. Results revealed that depending on the television material, on-screen user-generated comments either deter or enhance cognitive processing of the television content. Further, on-screen user-generated comments strengthen perceptions of social presence, while individual differences had little effect on that perception. Lastly, perceptions of social presence had an indirect positive effect on the relationship between on-screen user-generated comments and viewer experiences in one video stimulus, and an indirect negative effect on cognitive processing in the other video stimulus. These findings suggest that, while on-screen user-generated comments bring the ‘socialness’ back to television watching, they may be beneficial or detrimental based on the producer’s intent for the program.