When Pigs Fly is the tenth Pearls Before Swine book. It was released on September 7, 2010. When Pigs Fly contains strips from November 17, 2008 to August 23, 2009. All of the strips featured in When Pigs Fly were included in the treasury Pearls Blows Up.
When Pigs Fly also has The Unseen Pearls in it, a collection of 11 strips that were never published in newspapers.