Keith M. Carlson reviews the development of the monetarist econometric model that he and Andy published in the April 1970 Review article (reprinted beginning on p. 45) and presents an updated version, comparing it with the original model. The general form of the model, the author reports, remains unchanged; it has been simplified to a rate-of-change form, however, and some supply-side variables have been added. Carlson examines the properties of the updated model and summarizes its simulation performance. The properties of the model are little changed from the original Andersen-Carlson article: Monetary actions have substantial short-run effects on total spending, output, and unemployment, but over the long run, the effect on total spending is reflected almost entirely in the price level. Although the model’s simulation performance appears satisfactory, a more accurate evaluation awaits comparison with similar results from other models.