In the past fifteen years a new field of research has emerged in economics: the application of control theory methods to macroeconomics and to microeconomics. The papers and books which have resulted from this research are important to the development of theoretical and applied economics. However, they are inaccessible to many with interest in economics because of the technical nature of the discussion. This book attempts to make the macro­ economic portion of this literature more accessible by providing a discussion of the key issues using words and figures rather than mathematical symbols. I would like to thank my mentors and colleagues in control theory and economics for their help over the years: Masanao Aoki, Michael Athans, Yaakov Bar-Shalom, Jeremy Bray, Arthur Bryson, Gregory Chow, Ray Fair, Laurie Henrikson, David Livesey, Raman Mehra, Alfred Norman, Robert Pindyck, Franklin Shupp, John Taylor, Lance Taylor, Peter Tinsley, Edison Tse, and Stephen Turnovsky.