Public economics (or economics of the public sector) is the study of government policy through the lens of economic efficiency and equity. Public economics builds on the theory of welfare economics and is ultimately used as a tool to improve social welfare. Welfare can be defined in terms of well-being, prosperity, and overall state of being.

Public economics provides a framework for thinking about whether or not the government should participate in economic markets and if so to what extent it should do so. Microeconomic theory is utilized to assess whether the private market is likely to provide efficient outcomes in the absence of governmental interference; this study involves the analysis of government taxation and expenditures.

This subject encompasses a host of topics notably market failures such as, public goods, externalities and Imperfect Competition, and the creation and implementation of government policy.

Broad methods and topics include:

Emphasis is on analytical and scientific methods and normative-ethical analysis, as distinguished from ideology. Examples of topics covered are tax incidence, optimal taxation, and the theory of public goods.

The Journal of Economic Literature (JEL) classification codes are one way categorizing the range of economics subjects. There, Public Economics, one of 19 primary classifications, has 8 categories. They are listed below with JEL-code links to corresponding available article-preview links of The New Palgrave Dictionary of Economics Online (2008) and with similar footnote links for each respective subcategory if available:

The role of government in providing efficient and equitable markets is largely underpinned by addressing market failures that may arise. Public Economics focuses on when and to what degree the government should intervene in the economy to address market failures. Some examples of government intervention are providing pure public goods such as defense, regulating negative externalities such as pollution and addressing imperfect market conditions such as asymmetric information.

Pure public goods, or collective consumption goods, exhibit two properties; non-rivalry and non-excludability. Something is non-rivaled if one person's consumption of it does not deprive another person, (to a point) a firework display is non-rivaled - since one person watching a firework display does not prevent another person from doing so. Something is non-excludable if its use cannot be limited to a certain group of people. Again, since one cannot prevent people from viewing a firework display it is non-excludable. Due to these constraints, one of few examples of a "pure public good" is national defense - it is both non-rivalry and non-excludable. Another example, of a pure public good is knowledge. Consider a book. The book itself can be destroyed and thus is excludable. However, the knowledge obtained from the book is far more difficult to destroy and is non-rivalrous and non-excludable. In reality, not all public goods can be classed as 'pure' and most display some degree of excludability and rivalrous. These are known as Impure public goods. To visualize the public good's characteristic of non-excludability, it would be the inability to build a fence, barrier or wall that would block the good from consumption. In the modern era, digital replication allows several goods to be non-rivalry; since, people from all over the world can access it if you have access to the internet and a device.