Lurking, or passive online participation, is often defined as non-participation or minimal participation in online groups or communities. Although lurking has previously been considered a negative behavior in online spaces, with lurkers accessing the social capital of online communities without providing anything in return, current perspectives consider lurking as a legitimate form of online participation. Current literature shows that lurking is a more nuanced activity than previously considered, as individuals may not only vary in their participation across different online groups but their participation may also vary over time within an online group. This chapter examines the key personal and situational factors associated with active or passive participation, as well as the differential outcomes linked to levels of engagement in online groups generally, followed by a more detailed exploration of lurking in the context of online support groups and education.