It is a day to celebrate books and reading and school children typically dress up as their favourite book character.
Have a look at some of the children in Derry dressed up today.
5. Spider Girl
Clára, age 3, dressed as Spider Girl for her first World Book Day.
6. Peter Rabbit and Sir Alex Ferguson
Cormac as Peter Rabbit and Cathal as Sir Alex Ferguson
7. Ryan McBride
Ethan as Ryan McBride, Number 5.
8. Matilda
Evangeline, aged 6, as Matilda.