This article provides an overview of difference-in-differences estimation, starting with a review of the basic methodology, discussing in some detail recent advances in inference, and concluding with new methods for estimating treatment effects in various nonlinear and semiparametric models.

Many empirical questions in economics and other social sciences depend on causal effects of programs or policies. In the last two decades much research has been done on the econometric and statistical analysis of the effects of such programs or treatments. This recent theoretical literature has...

This paper proposes a method to estimate treatment effects in difference-in-differences designs in which the treatment start is staggered over time and treatment effects are heterogeneous by group, time, and covariates, and when the data are repeated cross-sections. We show that a...