This video is the third in our videos that review what we have done in Econ 101. It focuses on our treatment of macroeconomics in the short, intermediate and long run and concludes with a review of how ideas from causal inference have run throughout the course as we try to test our models. While many of our examples of causal inference have been of the reduced form kind, we emphasize that macroeconomics often relies on a structural approach to causal inference.