Interpersonal communication often takes place in groups: groups of friends, family members, sport teams, or work teams. The same holds true for mediated interpersonal communication-a great deal of it takes place in virtual groups or virtual communities. Horrigan et al. (2001) report that 84 percent of American internet users have contacted an online group at least once. This chapter focuses on virtual communities-on the wellestablished ones as well as on new trends.