In 1984, Robert Axelrod published a book, relating the story of two competitions which he ran, where invited academics entered strategies for the Iterated Prisoners' 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 prisoners' dilemma strategies. The first of these new competitions was run in July 2004, and the second in April 2005. Iterated Prisoners' Dilemma: 20 Years On essentially provides an update of the Axelrod's book.Specifically, it Presents the prisoners' dilemma, its history and variants;Highlights original Axelrod's work and its impact;Discusses results of new competitions;Showcases selected papers that reflect the latest researches in the area;