1 Introduction
2 Background
2.1 Abstractive Summarization Methods
2.2 Factual Inconsistency Error
3 Factual Consistency Metrics
3.1 Unsupervised Metrics
3.2 Weakly Supervised Metrics
3.3 Meta Evaluation
4 Factual Consistency Optimization
4.1 Fact-Encode-Based
4.2 Textual-Entailment-Based
4.3 Post-Editing-Based
4.4 Other
5 Conclusion and Future Directions