1) Implicit user interest modeling learns users' potential interests that were not explicitly mentioned by analyzing relationships between users and topics of interest. This approach is important for passive users who do not generate much content. 2) Relationships between users, such as followers on Twitter, and relationships between topics, such as hierarchies in Wikipedia, can provide clues about a user's implicit interests. A user's interests may be semantically related or similar to the interests of other users they follow or topics that are related in a knowledge base. 3) Techniques like collaborative filtering and frequent pattern mining can identify implicit interests by measuring relatedness between a user's explicit interests and other users' interests or topics in a collection.