This survey gives a brief overview of the literature on the difference-in-difference (DiD) estimation strategy and discusses major issues using a treatment effect perspective. In this sense, this survey gives a somewhat different view on DiD than the standard textbook discussion of the difference… -in-difference model, but it will also not be as complete as the latter. This survey contains also a couple of extensions to the literature, for example, a discussion of and suggestions for non-linear DiD as well as DiD based on propensity-score type matching methods.