In 1984, Robert Axelrod published a book, relating the story of two competitions which he ran, where invited academics entered strategies for the Iterated PrisonersOCO Dilemma. The book, almost 20 years on, is still widely read and cited by academics and the general public. As a celebration of that landmark work, we have recreated those competitions to celebrate its 20th anniversary, by again inviting academics to submit prisonersOCO dilemma strategies. The first of these new competitions was run in July 2004, and the second in April 2005. Iterated PrisonersOCO Dilemma: 20 Years On essentially provides an update of the AxelrodOCOs book. Specifically, it. OCo Presents the prisonersOCO dilemma, its history and variants. OCo Highlights original Axelrod's work and its impact. OCo Discusses results of new competitions. OCo Showcases selected papers that reflect the latest researches in the area."