Litter consists of waste products that have been discarded incorrectly, without consent, at an unsuitable location. The waste are objects, often man-made, such as aluminum cans, paper cups, food wrappers, cardboard boxes or plastic bottles, but also large and hazardous items of rubbish such as tires, electrical appliances, electronics, batteries and large industrial containers are sometimes dumped in isolated locations, such as national forests and other public lands.

Litter is a type of human impact on the environment and is a serious environmental problem in many countries. Litter can exist in the environment for long periods of time before decomposition and be transported over large distances into the world's oceans. Litter can negatively affect quality of life.

Cigarette butts are the most littered item in the world, with an estimated 4.5 trillion discarded each year. Estimates of the required time for cigarette butts to break down vary, ranging from 5 to 400 years for complete degradation.

In addition to intentional littering, almost half of litter on U.S. roadways is now accidental or unintentional litter, usually debris that falls off improperly secured trash, recycling collection vehicles and pickup trucks. Population levels, traffic density and proximity to waste disposal sites are factors known to correlate with higher litter rates.

Illegally dumped hazardous waste may be a response to the costs of dropping materials at designated sites: some of these charge a fee for depositing hazardous material. Lack of access to nearby facilities that accept hazardous waste may deter use. Additionally, ignorance of the laws that regulate the proper disposal of hazardous waste may cause improper disposal.

According to a study by the Dutch organization VROM, 80% of people claim that "everybody leaves a piece of paper, tin or something, on the street behind". Young people from 12 to 24 years cause more litter than the average (Dutch or Belgian) person; only 18% of people who regularly cause litter were 50 years of age or older. However, a 2010 survey of littering in Maine, New Hampshire and Vermont in the United States, placed litterers aged 55 and over at less than 5%. The same observational study estimated that 78% of litterers are male. Litter organizations, such as Keep America Beautiful affiliates, Keep Northern Ireland Beautiful, and the Bay Area Stormwater Management Agencies Association, have focused educational efforts on youth littering.

Negligent or lenient law enforcement contributes to littering behavior. Other causes are inconvenience, entitlement and economic conditions. A survey of dumping in Pennsylvania found that the largest number of illegal dumps were in townships without municipal trash hauling. The same report also cites unavailability of curbside trash and recycling service, shortage of enforcement, and habit as possible causes. The presence of litter invites more littering.

The two-stage process model of littering behavior describes the different ways in which people litter. The model was proposed by Chris Sibley and James Liu and differentiates between two types of littering: active and passive.