Reputation is a social control artefact developed by human communities to encourage socially desirable behaviour in absence of a central authority. It is widely employed in online contexts to address a number of dilemmas that the interaction among strangers can raise. This chapter presents a social-cognitive theory as a framework to describe the dynamics of reputation formation and spreading. In Section 2, we examine the technology of reputation as implemented in some popular Web platforms, testing theory predictions about the tendency towards either a rule of courtesy or a rule of prudence in evaluation reporting, and thus trying to better understand the outcomes that each system promotes and inhibits. Â© 2010, IGI Global.