In this paper we give an overview of the Knowledge Base Population (KBP) track at TAC 2011. The main goal of KBP is to promote research in discovering facts about entities and expanding a structured knowledge base with this information. Compared to KBP2010, we extended the Entity Linking task to require clustering of mentions of entities not already in the KB (‘NIL queries’). We also introduced two new tasks-(1) Cross-lingual Entity Linking: Given a set of multi-lingual (English and Chinese) queries, the system is required to provide the ID of the English KB entry to which each query refers and cluster NIL queries without KB references; and (2) Temporal Slot Filling: given an entity query, the system is required to discover the start and end dates for any identified slot fill. KBP2011 has attracted many participants (over 65 teams registered, among which 35 teams submitted results). In this paper we provide an overview of the task definition, annotation issues, successful methods and research challenges associated with each task in KBP2011.