Descriptions of how to design an experimental study in the social sciences are numerous, but here we address the basic concepts as they apply to L2 writing research. Simply put, experimental research attempts to control and manipulate variables to determine cause-effect relationships. The variables that are manipulated are the independent variables (also called the treatment or intervention), while the outcome variables are the dependent variables . As an example from L2 writing research, consider a study by van Beuningen, De Jong, and Kuiken (2012). The authors wanted to determine the effect of comprehensive written corrective feedback, a type of treatment, on accuracy in writing. The independent variable in this case is the feedback, and accuracy is the dependent variable. In addition to research on corrective feedback, other purposes for conducting experimental research are first addressed in this chapter. This is followed by a discussion of the central issues related to experimental research and the discussion of a sample study.