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Resources that, because of their “public” nature, are difficult or costly to exclude anyone from using. Examples include lighthouses, city parks, broadcast programming and the global atmosphere. In the lingo of economists, these are “non-rival” and “non-excludable” resources. Government often steps in to pay for public goods because it is difficult to get individual beneficiaries to pay for them. But in the networked environment of the Internet, it is increasingly feasible for self-organizing groups to create and pay for public goods. Open source software is a prime example.

== In economics theory ==

In economics, a public good is a good that is non-rivalrous and non-excludable. This means that consumption of the good by one individual does not reduce availability of the good for consumption by others; and that no one can be effectively excluded from using the good. In the real world, there may be no such thing as an absolutely non-rivaled and non-excludable good; but economists think that some goods approximate the concept closely enough for the analysis to be economically useful.

For example, if one individual visits a doctor there is one less doctor's visit for everyone else, and it is possible to exclude others from visiting the doctor; it is a rivaled and excludable private good. Conversely, breathing air neither significantly reduces the amount of air available to others, nor can people be effectively excluded from using the air. This makes it a public good, but one that is economically trivial, as air is a free good. A less straight-forward example is the exchange of MP3 music files on the internet: the use of these files by any one person does not restrict the use by anyone else and there is little effective control over the exchange of these music files and photo files.

Non-rivalness and non-excludability may cause problems for the production of such goods. Specifically, some economists[by whom?] have argued that they may lead to instances of market failure, where uncoordinated markets driven by parties working in their own self interest are unable to provide these goods in desired quantities. These issues are known as public goods problems, and there is a good deal of debate and literature on how to measure their significance to an economy, and to identify the best remedies. These debates can become important to political arguments about the role of markets in the economy. More technically, public goods problems are related to the broader issue of externalities.

Graphically, non-rivalry means that if each of several individuals has a demand curve for a public good, then the individual demand curves are summed vertically to get the aggregate demand curve for the public good . This is in contrast to the procedure for deriving the aggregate demand for a private good, where individual demands are summed horizontally.

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