1 INTRODUCTION  
2 BACKGROUND  
2.1 Notation  
2.2 Learning Paradigms  
2.3 Scope of the Survey  
3 M ULTIMODAL L EARNING WITHOUT L ABELS  
4 MODALITY FUSION  
4.1 Fusion during Pre-training  
4.2 Fusion by Stitching  
5 UNALIGNED MULTIMODAL LEARNING  
5.1 Coarse-grained  
5.2 Fine-grained  
6 THEORETICAL CONSIDERATIONS  
7 APPLICATIONS  
7.1 State Representation Learning for Control  
7.2 Healthcare  
7.3 Remote Sensing  
8 CHALLENGES AND FUTURE DIRECTIONS