1 Introduction  
1.1 Developing of NLG  
1.2 The Faithfulness Problem  
1.3 Structure of This Survey  
2 Problem Analysis  
2.1 Definition and Categorization  
2.2 Challenges and Issues  
2.3 Cause Analysis  
3 Automatic Evaluation Metrics  
3.1 Entailment-based Metrics  
3.2 QA-based Metrics  
3.3 Fact-based Metrics  
3.4 Other Metrics  
3.5 Meta Evaluation  
4 Optimization Methods  
4.1 Faithfulness in Abstractive Summarization  
4.2 Faithfulness in Dialogue Generation  
4.3 Faithfulness in Machine Translation  
4.4 Faithfulness in Data-to-Text Generation  
4.5 Faithfulness in Other NLG Tasks  
5 Discussion  
5.1 Fine-grained and General Evaluation  
5.2 Reasoning-based Optimization