Season 21

Season 21 of The Simpsons aired from September 27, 2009 to May 23, 2010. It was the first season to air entirely in high-definition.

With this season, The Simpsons established itself as the longest-running American primetime television series, replacing , which consisted of 20 seasons.

In 2009, to celebrate the 20th anniversary of the premiere of The Simpsons, Fox announced a year-long celebration of the show titled "Best. 20 Years. Ever". It run from January 14, 2009 to January 14, 2010 and it included the documentary The Simpsons 20th Anniversary Special – In 3-D! On Ice!.

Reception
The season received mainly positive reviews from critics, with the most praised episodes being "The Squirt and the Whale", "To Surveil with Love" and "The Bob Next Door". Some episodes made the series receive many award nominations, winning two: an for Anne Hathaway for her voicing of Princess Penelope in "Once Upon a Time in Springfield" and an  for "Treehouse of Horror XX".