This paper will introduce the concept of republicanism and specifically the central republican principle of the common good ("res publica"). Republicanism is the doctrine that government and society should be structured and maintained for the common welfare of all, including everyone and excluding none. Other republican principles such as liberty, virtue, justice, dignity, humanism, morality, and the rule of law follow from this first commitment. So do the vices condemned by republicanism, such as domination, corruption, avarice, and ambition. So (for example) to enjoy liberty is to live in a polity in which no one is regulated or controlled, except by rules designed to advance the common good. To be virtuous is to work to advance the common good and respect shared standards that serve the public welfare. The republican form of government is that constitution best calculated to make the republic real. By the same token to coerce another without any public-regarding justification is domination. To use public power to pursue private ends is corruption, and so forth. Republican values offer a universal standard of law and morality, and exist, in one form or another, in every legal culture.