He introduced the concept of Pareto efficiency and helped develop the field of microeconomics. He also was the first to discover that income follows a Pareto distribution, which is a power law probability distribution. The Pareto principle was named after him and built on observations of his such as that 80% of the land in Italy was owned by 20% of the population. He also contributed to the fields of sociology and mathematics.
Pareto turned his interest to economic matters and he became an advocate of free trade, finding himself in difficulty with the Italian government. His writings reflected the ideas of Léon Walras that economics is essentially a mathematical science. Pareto was a leader of the "Lausanne School" and represents the second generation of the Neoclassical Revolution. His "tastes-and-obstacles" approach to general equilibrium theory were resurrected during the great "Paretian Revival" of the 1930s and have influenced theoretical economics since.
Pareto realized that cardinal utility could be dispensed with—that is, it was not necessary to know how much a person valued this or that, only that he preferred X of this to Y of that. Utility was a preference-ordering. With this, Pareto not only inaugurated modern microeconomics, but he also demolished the alliance of economics and utilitarian philosophy (which calls for the greatest good for the greatest number; Pareto said "good" cannot be measured). He replaced it with the notion of Pareto-optimality, the idea that a system is enjoying maximum economic satisfaction when no one can be made better off without making someone else worse off. Pareto optimality is widely used in welfare economics and game theory. A standard theorem is that a perfectly competitive market creates distributions of wealth that are Pareto optimal.