These lectures in macroeconomics explain the theory and policy implications of macroeconomics in a systematic and logically consistent way. Central to this analysis is the principle of aggregate demand as formulated by the Polish economist Michał Kalecki, who is best known as the originator, along with Keynes, of the Keynesian Revolution in macroeconomics. The lectures cover the main components of aggregate demand, showing the key importance of firms’ investment for total output, employment, and economic growth in both closed and open economies. The main influences on investment are explained and how, through the circular flow of income and expenditure, investment generates profits in the economy. However, investment is unstable and the government therefore has a central role in stabilizing such an economy at high rates of employment. Along with investment, the labor market and wages then determine the distribution of income. This leads on to an examination of the role of money and finance in the contemporary capitalist economy. The analysis is illustrated with statistics and a survey of the evolution of capitalist economies since World War II, along with critical observations on the neoclassical approach to economics.