This book is a practical guide to building economic models, both macroeconomic and multisectoral. It uses free software available from the Internet together with regularly updated databanks including the quarterly national accounts of the United States. It does NOT deal with some recent fads in economic model building, such as Real Business Cycles, Computable General Equilibriium models, nor DSGE models, all of which are, in the author's opinion, "alternative reality" models at best.