1 INTRODUCTION

2 BACKGROUND  
2.1 Formalization of Domain Generalization  
2.2 Related Research Areas  

3 THEORY  
3.1 Domain Adaptation  
3.2 Domain Generalization  

4 METHODOLOGY  
4.1 Data Manipulation  
4.2 Representation Learning  
4.3 Learning Strategy  

5 OTHER DOMAIN GENERALIZATION RESEARCH AREAS  
5.1 Single-source Domain Generalization  
5.2 Semi-supervised Domain Generalization  
5.3 Federated Learning with Domain Generalization  
5.4 Other DG Settings  

6 APPLICATIONS  

7 DATASETS, EVALUATION, AND BENCHMARK  

8 DISCUSSION  
8.1 Summary of Existing Literature  
8.2 Future Research Challenges  

9 CONCLUSION