 As we will see in Section 1, the economic definition of a public good has little to do with whether these goods are provided by the public or by private enterprises but with certain abstract features that are shared by many different goods, only some of which are regularly produced publicly. The abstract features economists use in their definition depend on technology, values and tastes, making boundaries contested and shifting over time. Section 2 will introduce the notion of an externality, and in Section 3 the standard neoclassical welfare economic analysis of public goods will be examined. Section 4 looks at the private provision of public goods. Section 5 offers a review of recent experimental work on public goods (which challenges the standard analysis to some extent). Section 6, finally, discusses some ethical arguments relevant to the provision of public goods.