The two camps of economists have different views concerning how to improve the economy in times of economic downturn. Representatives of the first camp do not believe that a large national debt will inevitably undermine economic growth and can even throw the economy into recession. They consider government spending on infrastructure as an efficient strategy and support the approach based on additional government borrowing. Another group of economists that concerns with high government debt which, as they believe, can inevitably undermine economic growth, supports austerity measures. Economists belonging to this camp consider the solution of the huge national debt problem as an urgent task. They believe that the approach based on additional government borrowing with a hope that this will help decrease the debt in the future has less probability of success than immediate austerity measures. The authors hope that their book will add insight to this debate. The developed models relate to the two approaches used to improve the economy in periods of economic downturn. The important issue is discussed: whether it is better to let debt increase in the hope of stimulating economic growth to get out of the slump or cut spending to get national debt under control. Austerity and spending are very hot topics discussed by economists, politicians, and journalists. Unfortunately, there is no an agreement concerning a proper fiscal policy to improve the economy. The book shows how to use the developed models to evaluate the dynamics of the debt to GDP ratio for the cases of government spending and tax cuts and make decision whether such economic measures are efficient. It also offers an approach to establish yearly goals to decrease a country's debt based on a long-term goal and balance its budget in a certain number of years. The book contains concrete examples related to the U.S. economy and computer programs used to solve these examples; these programs can be used to analyze the economies of other countries. Basic facts about stimulus and austerity policies are given in Chapter 1. The system approach is used to analyze these policies. Their main goals and related macroeconomic parameters of economic models are discussed. Special attention is paid to the debt to GDP ratio, which is considered as a compromised criterion to evaluate stimulus and austerity policies. Chapter 2 contains the debt to GDP ratio dynamics models developed to forecast the evolution of debt to GDP ratio over a 10-years horizon and evaluate the efficiency of government stimulus policy.Austerity policy is discussed in Chapter 3. The results of optimal theory are used to obtain optimal yearly debt levels that should be realized by an appropriate fiscal policy of the government. A more moderate policy of balancing budget by a specific year is also discussed. In Chapter 4 the theoretical results of Chapter 2 and Chapter 3 are used to evaluate the implemented economic policies. Comparative analysis of the results, based on the developed models, with the recommendations used by the U.S. government during the 2008 financial crisis and expectations, which were not well founded, is presented. . Several approaches that can improve the economy are considered. Chapter 5 contains software developed to solve multiple examples presented in the previous chapters. It can be useful for researchers who would test the considered models and use them in practice.