In the physical world, we express and perceive identity through numerous cues. We learn about people by observing where they are, who they are with, what they are wearing, how they speak, etc. In the online world, most of these familiar cues are missing. There are, however, other cues, such as the history of a person's contributions to a public forum or the set of acquaintances they are linked to in a communication web. In this seminar we will first review some major approaches to identity perception, focussing on prototype models and the problem of cue reliability. We will then look at ways identity is expressed online, analysing whether and if so, how the differences between the physical and mediated worlds would lead to fundamentally different formulations of identity. Finally, we will look at the role of visualizations in making these identity cues visible.