Mixed-sex education, also known as mixed-gender education, co-education, or coeducation (abbreviated to co-ed or coed), is a system of education where males and females are educated together. Whereas single-sex education was more common up to the 19th century, mixed-sex education has since become standard in many cultures, particularly in western countries. Single-sex education remains prevalent in many Muslim countries. The relative merits of both systems have been the subject of debate.

The world's oldest co-educational school is thought to be Archbishop Tenison's Church of England High School, Croydon, established in 1714 in the United Kingdom, which admitted boys and girls from its opening onwards. This has always been a day school only.

The world's oldest co-educational both day and boarding school is Dollar Academy, a junior and senior school for males and females from ages 5 to 18 in Scotland, United Kingdom. From its opening in 1818, the school admitted both boys and girls of the parish of Dollar and the surrounding area. The school continues in existence to the present day with around 1,250 pupils.

The first co-educational college to be founded was Oberlin Collegiate Institute in Oberlin, Ohio. It opened on 3 December 1833, with 44 students, including 29 men and 15 women. Fully equal status for women did not arrive until 1837, and the first three women to graduate with bachelor's degrees did so in 1840. By the late 20th century, many institutions of higher learning that had been exclusively for men or women had become coeducational.

In early civilizations, people were typically educated informally: primarily within the household. As time progressed, education became more structured and formal. Women often had very few rights when education started to become a more important aspect of civilization. Efforts of the ancient Greek and Chinese societies focused primarily on the education of males. In ancient Rome, the availability of education was gradually extended to women, but they were taught separately from men. The early Christians and medieval Europeans continued this trend, and single-sex schools for the privileged classes prevailed through the Reformation period. The early periods of this century included many religious schools and the first major public schools in the country had been established for males and females.

In sharp contrast, in the Muslim world, females played prominent roles in education from the beginning of Islamic history. The wife of the prophet, Aisha, turned her home into a center of learning where both genders flocked to for classes. Umm al-Darda in the 7th century used to study in both men's and women's circles and then became a prominent teacher herself, even teaching at the Dome of the Rock in Jerusalem, and the caliph Abd al-Malik ibn Marwan was her student. Many prominent historic Muslim jurists were educated by female scholars, including the famed imam Al-Shafi'i by Sayyida Nafisa. The book Al-Wafa bi Asma al-Nisa is devoted to female hadith scholars alone, and covers over 10,000 women in Islamic history. The various halaqas in the Muslim world through the centuries were often open to both genders, and their stories are visible in numerous historic records. Two examples are Shaykhah Umm Al-Khayr Fatimah bint Ibrahim and her contemporary Sitt Al-Wuzara who taught both men and women in prominent mosques in the 14th century CE. However, as the centuries passed, an interesting phenomenon is observed--the slowly diminishing role of women in education as the empires spread and absorbed non-Muslim cultures, such as the Byzantines and Sassanians and more, whose pre-Islamic cultures had a long history of patriarchy and were sometimes reticent to adjust to the new Islamic norms. Nonetheless, women's roles in education were, still yet, incomparably pronounced compared to any other premodern civilization in human history by a massive margin.

In the 16th century, at the Council of Trent, the Roman Catholic church reinforced the establishment of free elementary schools for children of all classes. The concept of universal elementary education, regardless of sex, had been created. After the Reformation, coeducation was introduced in Western Europe, when certain Protestant groups urged that boys and girls should be taught to read the Bible. The practice became very popular in northern England, Scotland, and colonial New England, where young children, both male and female, attended dame schools. In the late 18th century, girls gradually were admitted to town schools. The Society of Friends in England, as well as in the United States, pioneered coeducation as they did universal education, and in Quaker settlements in the British colonies, boys and girls commonly attended school together. The new free public elementary, or common schools, which after the American Revolution supplanted church institutions, were almost always coeducational, and by 1900 most public high schools were coeducational as well. In the late 19th and early 20th centuries, coeducation grew much more widely accepted. In Great Britain, Germany, and the Soviet Union, the education of girls and boys in the same classes became an approved practice.

In Australia, there is a trend towards increased coeducational schooling with new coeducational schools opening, few new single-sex schools opening and existing single-sex schools combining or opening their doors to the opposite gender.