A sit-in or sit-down is a form of direct action that involves one or more people nonviolently occupying an area for a protest, often to promote political, social, or economic change.

In sit-ins, protesters usually seat themselves at some strategic location (inside a restaurant, in a street to block it, in a government or corporate office, and so on). They remain until they are evicted, usually by force, or arrested, or until their requests have been met. Sit-ins have historically been a highly successful form of protest because they cause disruption that draws attention to the protest and by proxy the protesters' cause. They are a non-violent way to effectually shut down an area or business. The forced removal of protesters, and sometimes the use of violence against them, often arouses sympathy from the public, increasing the chances of the demonstrators reaching their goal.

Sit-ins were an integral part of the nonviolent strategy of civil disobedience and mass protests that eventually led to passage of the Civil Rights Act of 1964 which ended legally-sanctioned racial segregation in the United States and also passage of the Voting Rights Act of 1965 that struck down many racially-motivated barriers used to deny voting rights to non-whites.

Sitting is a rest position supported by the buttocks or thighs where the torso is more or less upright.

The most common way of sitting on the floor involves bending the knees. One can also sit with the legs unbent, using something solid as support for the back or leaning on one's arms.

Sitting with bent legs can be done along two major lines; one with the legs mostly parallel and one where they cross each other. The parallel position is reminiscent of, and is sometimes used for, kneeling. The latter is a common pose for meditating.

Most raised surfaces at the appropriate height can be used as seats for humans, whether they are made for the purpose, such as chairs, stools and benches, or not. While the buttocks are nearly always rested on the raised surface, there are many differences in how one can hold one's legs and back.

There are two major styles of sitting on a raised surface. The first has one or two of the legs in front of the sitting person; in the second, sitting astride something, the legs incline outwards on either side of the body.