Trust-based recommender systems are a class of recommender systems that incorporate trust relationships between users into the recommendation process. These systems operate on the principle that users are more likely to accept recommendations from other users they trust. Trust-based recommender systems have been proven more effective in providing accurate recommendations than trust-unaware recommender systems. This work presents a comprehensive, up-to-date survey about trust-based recommender systems in which we carefully analyze different approaches and techniques. We discuss the incorporation of trust in recommendation, classify the techniques used in the literature according to a taxonomy, and highlight open issues in the field.