The Glorious Twelfth is the twelfth day of August, the start of the shooting season for red grouse (Lagopus lagopus scotica), and to a lesser extent the ptarmigan (Lagopus muta) in Great Britain and Northern Ireland. This is one of the busiest days in the shooting season, with large amounts of game being shot. The date itself is traditional; the current legislation enshrining it in England and Wales is the Game Act 1831 (and in Northern Ireland, the Wildlife (Northern Ireland) Order 1985). Not all game (as defined by the 1831 act) have the same start to their open seasons – most begin on 1 September, with 1 October for woodcock and pheasant.
Since English law prohibits game bird shooting on a Sunday, the start date is postponed to 13 August on years when the 12th falls on a Sunday.  Because grouse are not and never have been reared to any extent for shooting, their numbers fluctuate naturally from year to year. In recent years, the Glorious Twelfth has also been hit by hunt saboteurs, the 2001 foot and mouth crisis (which further postponed the date in affected areas) and the effect of sheep tick, heather beetle, the gut parasite Trichostrongylus tenuis and severe flooding and bad weather. In some seasons where certain moors are hit by low numbers of grouse, shooting may not occur at all or be over by September.
- Driven grouse shooting
- Hunting in the United Kingdom
- British Association for Shooting and Conservation
- "Wild birds: management and legal protection". DEFRA. 2013-06-26. Retrieved 2013-08-14.
- Kirsty Scott (2001-08-13). "Glorious Twelfth hit by foot and mouth". London: Guardian. Retrieved 2010-12-26.
- "'Glorious 13th' for grouse season". BBC News. 2007-08-13. Retrieved 2010-12-26.
- Clover, Charles (2001-05-08). "Glorious Twelfth cancelled in areas affected by virus". The Daily Telegraph. Retrieved 2012-05-09.
- "Grouse season 'not so glorious'". BBC News. 2004-08-12. Retrieved 2010-12-26.