Market failure is a concept within economic theory describing when the allocation of goods and services by a free market is not efficient. That is, there exists another conceivable outcome where a market participant may be made better-off without making someone else worse-off. (The outcome is not Pareto optimal.) Market failures can be viewed as scenarios where individuals' pursuit of pure self-interest leads to results that are not efficient – that can be improved upon from the societal point-of-view. The first known use of the term by economists was in 1958, but the concept has been traced back to the Victorian philosopher Henry Sidgwick.

Market failures are often associated with information asymmetries,non-competitive markets, principal–agent problems, externalities, or public goods. The existence of a market failure is often used as a justification for government intervention in a particular market. Economists, especially microeconomists, are often concerned with the causes of market failure and possible means of correction. Such analysis plays an important role in many types of public policy decisions and studies. However, some types of government policy interventions, such as taxes, subsidies, bailouts, wage and price controls, and regulations, including attempts to correct market failure, may also lead to an inefficient allocation of resources, sometimes called government failure. Thus, there is sometimes a choice between imperfect outcomes, i.e. imperfect market outcomes with or without government interventions. But either way, if a market failure exists the outcome is not pareto efficient. Mainstream neoclassical and Keynesian economists believe that it may be possible for a government to improve the inefficient market outcome, while several heterodox schools of thought disagree with this.