1 Introduction  
2 TWFE regressions with heterogeneous treatment effects  
2.1 TWFE regressions may not identify a convex combination of treatment effects  
2.2 The origin of the problem: “forbidden comparisons”  
2.3 Decomposition results for other TWFE regression coefficients  
3 Alternative heterogeneity-robust DID estimators  
3.1 Estimators ruling out dynamic effects  
3.2 Estimators allowing for dynamic effects when the treatment is binary and the design is staggered.  
3.3 Estimators allowing for dynamic effects when the treatment is not binary or the design is not staggered.  
4 Application  
5 Conclusion, and avenues for future research