Studying, developing, and applying theories and concepts of economics is no easy thing to do, and excellence in this is what it takes to become a good economist. If names like John M. Keynes, Adam Smith, Milton Friedman, David Hume, Ludwig von Mises, and Thomas Malthus feature in any given list of famous economists, it's because of their immense contribution. Though they came from different parts of the world, they had a key role in shaping the world economy. John Maynard Keynes was a British economist, best known for spearheading the revolution in economic thinking, which overturned the then existent ideas of neoclassical economics. Keynes is also regarded by many as the founder of modern theoretical macroeconomics. His ideas formed the basis of an independent school of thought, Keynesian economics. His contributions include some of the most useful economic concepts, including liquidity preference, fiscal multiplier, deficit spending, and aggregate demand-aggregate supply model. Adam Smith was a Scottish philosopher and political economist who is widely considered the father of modern economics and the world's first free-market capitalist, for he explained how―in a free-market economy―rational self-interest led to economic well-being. Undoubtedly one of the biggest names in this field, he influenced the likes of Karl Marx, John M. Keynes, and Thomas Malthus. Smith's contribution to economics came in the form of his famous works: The Theory of Moral Sentiments (1759) and The Wealth of Nations (1776); the latter was published as a five-book series. David Ricardo was a British political economist, who together with James Mill, founded classical economics. It was Adam Smith's The Wealth of Nations that inspired Ricardo to take interest in this subject. He was a believer of monetarism, which was evident from his argument that the propensity of the Bank of England to issue excess banknotes was responsible for the prevailing inflation in the country. His biggest contribution to the field of economics was his theory of comparative advantage. Yet another renowned figure in this field, Milton Friedman was an American economist and statistician. An ardent advocate of laissez-faire capitalism, Friedman was awarded the Nobel Memorial Prize in Economics in 1976. He was best known for his demonstration of the complexity of stabilization policy, which was introduced to stabilize the economy. Some of his prominent contributions to this field include the concept of monetarism, price theory, permanent income hypothesis, floating exchange rates, applied macroeconomics, and the Friedman test. Though he is more often remembered as a German philosopher and revolutionary sociologist, Karl Marx did a lot for economics as well. He was of the belief that humans were not motivated by grand ideas, but by material concerns related to survival. Marx looked at capitalism negatively and in his book Das Kapital argued that the capitalist's profits come from exploiting labor. He predicted the end of capitalism and emergence of communism, where people would own the means of production, and thus, there would be no need to exploit labor for profit. David Hume was a renowned philosopher and economist hailing from Scotland. Even though he was more of a philosopher, he is regarded as one of the most eminent economists of the world, as his discussions on politics led to the development of several ideas that are prevalent in the field of economics even today. He was of the opinion that foreign trade is a stimulus for economic growth, and therefore, is very important for the development of the nation. One of the famous American economists, Irving Fisher is best known for economic concepts such as the Fisher equation and Fisher separation theorem. It was his work on quantity theory of money that became the basis for the development of Milton Friedman's concept of 'monetarism'. Besides the Fisher equation and Fisher separation theorem, he is also known for contributions such as the equation of exchange, price index, Philips curve, and monetary illusion. Not many people would need an introduction to the English demographer and political economist, Thomas Robert Malthus; best known for his popularization of the economic theory of rent. He was one of those economists who played a crucial role in the development of classical economics as the first modern school of economic thought. Malthus was one of the most influential and controversial figures in the field of economics and politics. His major contribution to this field came in the form of the Malthusian growth model. Ludwig von Mises was one of the most important figures of the Austrian School of economics. He is best known for his work on praxeology, i.e., the deductive study of our actions which suggests that we engage in purposeful behavior. Mises' contribution to the world of economics came in the form of his works like The Theory of Money and Credit, Socialism: An Economic And Sociological Analysis, Human Action, and the Theory and History. Furthermore, his business cycle theories, which were quite influential in themselves, were developed by his student, Friedrich Hayek. Friedrich Hayek was an Austrian-born economist and philosopher, best known for his support of classical liberalism and free-market capitalism. A member of the Austrian School of economics, Hayek was awarded the Nobel Memorial Prize in Economics in 1974, which he shared with the Swedish economist, Gunnar Myrdal. The Hebbian Theory, economic calculation problem, price signal, spontaneous order, catallaxy, and dispersed knowledge were his major contributions to economics. Jean-Baptiste Say was a French economist, who is best known for the Say's Law, which suggests that supply creates its own demand. Despite its name, the Say's Law was not a product of his brain. He only popularized it in his work, the Treatise on Political Economy. Say was in favor of competition, free trade, and lifting restraints on business. Though he was responsible for introducing much of Adam Smith's work to continental Europe, he was also an ardent critic of some of Smith's noted work, such as the labor theory of value. British economist, Joan Robinson was one of the most famous women economists that the world has ever seen. She started off as a supporter of neoclassical economics, but eventually shifted to post-Keynesian economics. Robinson was best known for her knowledge about monetary economics and contributions to economic theory. Her major contributions to post-Keynesian economics came in the form of the Cambridge growth theory and Amoroso-Robinson relation. James Tobin was an American economist, who was one of the prominent figures involved in the development of Keynesian economics. He is best known for the Tobit model, which describes the relationship between a non-negative dependent variable and an independent variable. He was awarded the Nobel Memorial Prize in Economic Sciences in 1981. Tobin's noteworthy contributions to the field of macroeconomics include the modern portfolio theory, Tobin's q, Tobit model, Tobin tax, and the Mundell-Tobin effect. Amartya Sen, hailing from India, is best known for his human development theory, which is a combination of ecological economics, sustainable development, welfare economics, and feminist economics. Dr. Sen's contributions to welfare economics earned him the Nobel Prize in Economic Sciences in 1998. He was listed as one of the 100 most influential persons in the world by Time Magazine in 2010. Paul Krugman is an American economist best known for his work on international economics. It is not at all surprising that he features in this list; after all his noteworthy contributions include the international trade theory, new trade theory, new economic geography, etc. His work on the new trade theory and new economic geography earned him a Nobel Memorial Prize in Economics in 2008. A columnist for The New York Times, Krugman has several books and scholarly articles to his credit. Their work didn't just help the economy of their respective countries, but was also beneficial for the world economy. If it was not for these economists, several key concepts of the subject would have been a lot more difficult for us to understand or explain.