My main interest is in causal inference including  
Measurement error and misclassification in causal inference 
Principal stratification: noncompliance, surrogate, truncation by death
Causal mechanisms: mediation analysis, interaction, interference
Instrumental variable approaches: latent confounders, identifiability
Randomization-based analysis of experiments
Interference: spillover effect, contagious effect and infectiousness effect

I am also interested in missing data problems under non-ignorable missing data mechanisms:
Identification and partial identification 
Inference