Street children are poor or homeless children who live on the streets of a city, town, or village. Homeless youth are often called street kids, or urchins; the definition of street children is contested, but many practitioners and policymakers use UNICEF's concept of boys and girls, aged under 18 years, for whom "the street" (including unoccupied dwellings and wasteland) has become home and/or their source of livelihood, and who are inadequately protected or supervised. Street girls are sometimes called gamines, a term that is also used for Colombian street children of either sex.

Some street children, notably in more developed nations, are part of a subcategory called thrown-away children, consisting of children who have been forced to leave home. Thrown-away children are more likely to come from single-parent homes. Street children are often subject to abuse, neglect, exploitation, or, in extreme cases, murder by "clean-up squads" that have been hired by local businesses or police.

According to Consortium for Street Children, a street child has been defined as one "for whom the street (in the widest sense of the word, including unoccupied dwellings, wasteland, etc.) has become his or her habitual abode and/or source of livelihood; and who is inadequately protected, supervised, or directed by responsible adults".

Street children can be found in a large majority of the world's famous cities, with the phenomenon more prevalent in densely populated urban hubs of developing or economically unstable regions, such as some countries in Africa, the Americas, Eastern Europe, and Southeast Asia.

According to a report from 1988 of the Consortium for Street Children, a United Kingdom-based consortium of related non-governmental organizations (NGOs), UNICEF estimated that 100 million children were growing up on urban streets around the world. Fourteen years later, in 2002 UNICEF similarly reported, "The latest estimates put the numbers of these children as high as one hundred million". More recently the organization added, "The exact number of street children is impossible to quantify, but the figure almost certainly runs into tens of millions across the world. It is likely that the numbers are increasing." In an attempt to form a more reliable estimate, a statistical model based on the number of street children and relevant social indicators for 184 countries was developed; according to this model, there are 10 to 15 million street children in the world. Although it produced a statistically reliable estimate of the number of street children, the model is highly dependent on the definition of "street children," national estimates, and data collected on the development level of the country, and it is thus limited in range. The one hundred million figure is still commonly cited for street children, but is not based on currently available academic research. Similarly, it is debatable whether numbers of street children are growing globally, or whether it is the awareness of street children within societies that has grown.

Comprehensive street level research, completed in the year 2000 in Cape Town proved that international estimates of tens of thousands of street children living on the streets of Cape Town were incorrect. This research proved, that even with street children begging at every intersection, rivers of street children sleeping on the pavements at night, and with gangs of street children roaming around the streets, there were less than 800 children living on the streets of greater Cape Town at this time. This insight enabled a whole new approach to street children to be developed, one not based on the provision of basic care to masses of street children, but one focused on helping individual children, on healing, educating, stabilizing, and developing them permanently away from street life, as well as managing the exploitation of street children and the support factors that keep them on the street.

In 1848, Lord Ashley referred to more than 30,000 "naked, filthy, roaming lawless, and deserted children" in and around London, UK. Among many English novels featuring them as a humanitarian problem are Jessica's First Prayer by Sarah Smith (1867) and Georgina Castle Smith's Nothing to Nobody (1872).

By 1922, there were at least seven million homeless children in the Russian Soviet Federative Socialist Republic due to the devastation from World War I and the Russian Civil War (see "Orphans in the Soviet Union"). Abandoned children formed gangs, created their own argot, and engaged in petty theft and prostitution.