The upper class in modern societies is the social class composed of the wealthiest members of society, who also wield the greatest political power. According to this view, the upper class is generally contained within the wealthiest 1-2% of the population, and is distinguished by immense wealth (in the form of estates) which is passed on from generation to generation.. Prior to the 20th century, the nomenclature aristocracy was widely used. Out of the American population, one percent of the wealthiest population is responsible for thirty-four percent of the cumulative national wealth.

Because the upper classes of a society may no longer rule the society in which they are living, they are often referred to as the old upper classes and they are often culturally distinct from the newly rich middle classes that tend to dominate public life in modern social democracies. According to the latter view held by the traditional upper classes, no amount of individual wealth or fame would make a person from an undistinguished background into a member of the upper class as one must be born into a family of that class and raised in a particular manner so as to understand and share upper class values, traditions, and cultural norms. The term is often used in conjunction with terms like "upper-middle class," "middle class," and "working class" as part of a model of social stratification.

The social structure of the United Kingdom has historically been highly influenced by the concept of social class, with the concept still affecting British society in the early-21st century.

British society, like its European neighbours and most societies in world history, was traditionally (before the Industrial Revolution) divided hierarchically within a system that involved the hereditary transmission of occupation, social status and political influence. Since the advent of industrialisation, this system has been in a constant state of revision, and new factors other than birth (for example, education) are now a greater part of creating identity in Britain.

Although definitions of social class in the United Kingdom vary and are highly controversial, most are influenced by factors of wealth, occupation and education. Until recently the Parliament of the United Kingdom was organised on a class basis, with the House of Lords representing the hereditary upper class and the House of Commons representing everyone else. The British monarch is usually viewed as being at the top of the social class structure.