Every action taken on a social media platform is performed by a psychological being trying to satisfy some kind of psychological need-that is, individuals use social media to meet their emotional, social, attitudinal, cognitive, behavioral, and identity needs. Media systems that adapt to these needs (which have evolved over many millions of years) will be more successful than those that do not.  This tutorial will present a range of theories and recent empirical research in social, personality, and cognitive psychology to inform research and design on social media platforms. For example, self-verification theory predicts that much social interaction is motivated not by people’s need to be seen in a favorable light by others but by their needs to be seen by others as people see themselves; this prediction is supported by research examining perceptions of others based on their websites and Facebook profiles. Person-environment theories can be used to understand the processes of selection, evocation, and manipulation by which people use social media platforms to improve the fit between themselves and their social, physical, and virtual environments. The presentation will be geared towards drawing design implications for social media systems and for providing psychological frameworks to guide data collection about the individuals and groups that use social media.