1. Introduction  
2. Data Representations for Irregularly Sampled Time Series  
2.1 Series-Based Representation  
2.2 Vector-Based Representation  
2.3 Set-Based Representation  
3. Inference Tasks  
4. Modeling Primitives for Irregularly Sampled Time Series  
4.1 Discretization  
4.2 Interpolation  
4.3 Recurrence  
4.4 Attention  
4.5 Structural Invariance  
4.6 Summary  
5. Discretization-Based Approaches  
6. Interpolation-Based Approaches  
7. Recurrence-Based Approaches  
8. Attention-Based Approaches  
9. Structural Invariance-Based Approaches  
10. Predictive Performance  
11. Discussion and Directions for Future Work