He came up with the initial concept, developed the story, and co-wrote the screenplays for all four films:
* Raiders of the Lost Ark (1981): Lucas co-wrote the script with Lawrence Kasdan.
* Temple of Doom (1984): Lucas co-wrote the script with Willard Huyck and Gloria Katz.
* The Last Crusade (1989): Lucas co-wrote the script with Jeffrey Boam.
* Kingdom of the Crystal Skull (2008): Lucas co-wrote the script with David Koepp.
While the individual screenplays had other writers involved, Lucas was the driving force behind the overall story and characters of the Indiana Jones franchise.