A Sunday school is an educational institution, usually (but not always) Christian in character. Other religions including Buddhism, Islam, and Judaism have also organised Sunday schools in their temples and mosques, particularly in the West.

Sunday school classes usually precede a Sunday church service and are used to provide catechesis to Christians, especially children and teenagers, and sometimes adults as well. Churches of many Christian denominations have classrooms attached to the church used for this purpose. Many Sunday school classes operate on a set curriculum, with some teaching attendees a catechism. Members often receive certificates and awards for participation, as well as attendance.

Sunday school classes may provide a light breakfast. On days when Holy Communion is being celebrated, however, some Christian denominations encourage fasting before receiving the Eucharistic elements.