TripAdvisor. Airbnb. BlaBlaCar. We live in an online world increasingly powered by the star-ratings economy. Whenever we want to try a new restaurant, find a room for rent or catch a ride, we can rely on the user-generated content evaluating the place or service we’re looking for. So users can help each other out by sharing their information and experiences, in a continuously growing virtual village built on the rules of online reputation.In our everyday life, we are encouraged to virtually score every interaction we make – from the quality of a food delivery to a call centre service. And we tend to trust what we find in users’ ratings. But what is reputation, and why is it so important? Here at the Laboratory of Agent-Based Social Simulation we study reputation – including online reputation – as a social artifact, that shares with norms, responsibility, power, and trust the characteristic of making sense only in a social context. We have developed a computational and cognitive model of reputation, aiming at providing theory-driven and empirically backed up guidelines for designing reputation technology. We believe that reputation is a key ingredient for understanding society, and its study can help us finding a path in the complex and fascinating world of people’s opinions and beliefs.