Girls are denied access to education in many countries, particularly in the most marginalised communities.
The United Nations decided in 2011 that 11 October would be the International Day of the Girl Child after a two-year campaign by children's charity Plan.
This year, the theme of the event was innovation. Improving transport facilities, providing science and technology courses and using mobile technology in remote areas were all highlighted as ways in which girls could be helped to access education.
The Because I Am a Girl campaign, run by Plan, aims to equip schools, train teachers and fund scholarships. It also works with families to raise awareness of the dangers of early and forced marriage.
"Empowering girls, ensuring their human rights and addressing the discrimination and violence they face are essential to progress for the whole human family," said UN secretary general Ban Ki-moon.
Teenager Malala Yousafzai, who was shot by the Taliban in Pakistan in 2012, is an outspoken campaigner for the right of girls and women to a quality education.
Women account for two-thirds of the world's 774 million adult illiterates*
Youth literacy rates**