In economics, an externality or external cost is an indirect cost or benefit to an uninvolved third party that arises as an effect of another party's activity. Externalities can be considered as unpriced components that are involved in either consumer or producer market transactions. Air pollution from motor vehicles is one example. The cost of air pollution to society is not paid by either the producers or users of motorized transport to the rest of society. Water pollution from mills and factories is another example. All (water) consumers are made worse off by pollution but are not compensated by the market for this damage. A positive externality is when an individual's consumption in a market increases the well-being of others, but the individual does not charge the third party for the benefit. The third party is essentially getting a free product. An example of this might be the apartment above a bakery receiving some free heat in winter. The people who live in the apartment do not compensate the bakery for this benefit.

Ronald Harry Coase was a British economist and author. Coase was educated at the London School of Economics, where he was a member of the faculty until 1951. He was the Clifton R. Musser Professor of Economics at the University of Chicago Law School, where he arrived in 1964 and remained for the rest of his life. He received the Nobel Memorial Prize in Economic Sciences in 1991.