The working class refers to a group of people in a social hierarchy, typically defined by earning wages or salaries through their ability to work. Members of the working class rely primarily upon earnings from wage labour. Most common definitions of "working class" in use in the United States limit its membership to workers who hold blue-collar and pink-collar jobs, or whose income is insufficiently high to place them in the middle class, or both. However, socialists define "working class" to include all workers who fall into the category of requiring income from wage labour to subsist; thus, this definition can include almost all of the working population of industrialized economies.

As with many terms describing social class, working class is defined and used in different ways. One definition used by many socialists is that the working class includes all those who have nothing to sell but their labour, a group otherwise referred to as the proletariat. In this sense, the working class includes white and blue-collar workers, manual and menial workers of all types, excluding individuals who derive their livelihood from business ownership or the labour of others.^{[verification needed]} The term, which is primarily used to evoke images of laborers suffering "class disadvantage in spite of their individual effort", can also have racial connotations, applying diverse themes of poverty and implications about whether one is deserving of aid.

In other contexts the term working class refers to a section of society dependent on physical labour, especially when compensated with an hourly wage (for certain types of science, as well as journalistic or political analysis). Working-class occupations can be categorized into four groups: unskilled labourers, artisans, outworkers, and factory workers.^{[page needed]}

Common alternative definitions of working class include definition by income level, whereby the working class is contrasted with a middle class on the basis of access to economic resources, education, cultural interests, and other goods and services, and the "white working class" has been "loosely defined" by the New York Times as comprising white people without college degrees.

Researchers in Australia have suggested that working class status should be defined subjectively as a self-identification with the working class group. This subjective approach allows individuals, rather than researchers, to define their own "subjective" and "perceived" social class.

Karl Marx defined the working class or proletariat as those individuals who sell their labour power for wages and who do not own the means of production. He argued that they were responsible for creating the wealth of a society, asserting that the working class physically build bridges, craft furniture, grow food, and nurse children, but do not own land or factories.

A sub-section of the proletariat, the lumpenproletariat (rag-proletariat), are the extremely poor and unemployed, such as day labourers and homeless people. Marx considered them to be devoid of class consciousness.

In Marxist terms wage labourers and those dependent on the welfare state are working class, and those who live on accumulated capital are not, and this broad dichotomy defines the class struggle. In The Communist Manifesto, Marx and Friedrich Engels argue that it is the destiny of the working class to displace the capitalist system, with the dictatorship of the proletariat (as opposed to the "dictatorship of the bourgeoisie") abolishing the social relationships underpinning the class system before then developing into a communist society in which "the free development of each is the condition for the free development of all."