1 Introduction  
2 Knowledge Sources  
2.1 Encyclopedic Knowledge  
2.2 Commonsense Knowledge  
3 Knowledge-Intensive NLP Tasks  
3.1 Overview of Knowledge-Intensive NLP Tasks  
3.2 Characteristics of Knowledge-Intensive Tasks  
4 Knowledge Fusion Methods  
4.1 Pre-Fusion Methods  
4.2 Post-Fusion Methods  
4.3 Hybrid-Fusion Methods  
4.4 Representative Models for Specific Tasks  
5 Challenges and Future Directions  
5.1 Unified PLMKEs Across Tasks and Domains  
5.2 Reliability of Knowledge Sources  
5.3 Reasoning Module Design  
6 Conclusions