A boarding school is a school where pupils live within premises while being given formal instruction. The word "boarding" is used in the sense of "room and board", i.e. lodging and meals. As they have existed for many centuries, and now extend across many countries, their functioning, codes of conduct and ethos vary greatly. Children in boarding schools study and live during the school year with their fellow students and possibly teachers or administrators. Some boarding schools also have day students who attend the institution by day and return off-campus to their families in the evenings.

Boarding school pupils are typically referred to as "boarders". Children may be sent for one year to twelve years or more in boarding school, until the age of eighteen. There are several types of boarders depending on the intervals at which they visit their family. Full-term boarders visit their homes at the end of an academic year, semester boarders visit their homes at the end of an academic term, weekly boarders visit their homes at weekends. There are also semi-boarders who attend a boarding school in the school hours for formal instruction and activities but return home by the end of the day. In some cultures, boarders spend the majority of their childhood and adolescent life away from their families. Boarding schools are relatively more prevalent in the United Kingdom (UK), India, China, and parts of Africa. These countries begin boarding schools at a very early age and for a longer span of time. However, boarding schools are relatively less prevalent in Europe and the US where it is mostly seen for grades seven or nine through grade twelve—the high school years. Some are for either boys or girls while others are co-educational. The United Kingdom has a long tradition of boarding school education and the term public school has an elite association. There are also some state boarding schools, many of which serve children from remote areas.

In some societies and cultures, boarding schools are the most elite educational option (such as Eton and Harrow, which have produced several prime ministers), whereas in other contexts, they serve as places to segregate children deemed a problem to their parents or wider society. Canada and the United States tried to assimilate indigenous children in the Canadian Indian residential school system and American Indian boarding schools respectively. Some function essentially as orphanages, e.g. the G.I. Rossolimo Boarding School Number 49 in Russia. Tens of millions of rural children are now educated at boarding schools in China. Therapeutic boarding schools offer treatment for psychological difficulties. Military academies provide strict discipline. Education for children with special needs has a long association with boarding; see, for example, deaf education and Council of Schools and Services for the Blind. Some boarding schools offer an immersion into democratic education, such as Summerhill School. Others are international, such as the United World Colleges.